astronomy

Dictionary: as·tron·o·my   (ə-strŏn'ə-mē)

1. The scientific study of matter in outer space, especially the positions, dimensions, distribution, motion, composition, energy, and evolution of celestial bodies and phenomena.
2. A system of knowledge or beliefs about celestial phenomena: the various astronomies of ancient civilizations.

[Middle English astronomie, from Old French, from Latin astronomia, from Greek astronomiā : astro-, astro- + -nomiā, -nomy.]

The study of the universe and the objects in it through scientific investigation. Since much of contemporary astronomy uses the laws and methods of physics, the terms “astronomy” and “astrophysics” are usually used interchangeably. However, modern astronomy also uses techniques from many other scientific disciplines, including chemistry, geology, and biology, for which the terms astrochemistry, planetary science, and astrobiology are increasingly used.

The use of geological knowledge and methods in analyzing close-up observations from spacecraft of planets and their satellites and of comets and asteroids closely links the disciplines of astronomy and planetary science. Indeed, the discovery of planets around distant stars holds for even closer relations in the future. Methods of studying molecules in interstellar clouds involve chemical knowledge. Planetary science and astrochemistry come together with astronomy in the search for life outside the solar system, part of the search for extraterrestrial intelligence (SETI). The National Aeronautics and Space Administration (NASA), the United States space agency, has placed a priority on astrobiology, including the investigation of Mars and the bringing of samples back to Earth from Mars. See also Asteroid; Comet; Cosmochemistry; Extraterrestrial intelligence; Interstellar matter; Planet; Planetary physics; Solar system.

Astronomers often lead in employing new technologies, pushing them to the limit in exploring extremely faint signals in various parts of the electromagnetic spectrum. Nearly all astronomical research is now heavily dependent on computers. Astronomical imagery is now dominated by light-sensitive silicon chips known as charge-coupled devices (CCDs), which are approximately 100 times more sensitive than film. Fiber optics are used for a variety of astronomical purposes, including the taking of hundreds of galaxy images simultaneously from the field of view of a telescope and bringing the light to a spectrograph that can produce simultaneous spectra of all the objects. The technology of active optics, in which the shape of a mirror is changed slightly at a high rate (often faster than 1 Hz) to compensate for the blurring of astronomical images caused by the Earth's atmosphere, is being increasingly pursued to eliminate the twinkling of stars. See also Adaptive optics; Fiber-optics imaging.

The opening of the 5-m (200-in.) Hale telescope at the Palomar Observatory on Palomar Mountain, California, in 1948 marked the beginning of a great period of development in optical astronomy. The light-gathering power of this telescope allowed cosmological study that extended most of the way to the beginning of time in the universe. It was joined in the task by several 4-m-class (160-in.) telescopes and by one less successful larger telescope. In the 1990s, new techniques of telescope making allowed the completion of several telescopes in the 10-m (400-in.) class, twice the diameter and thus four times the collecting area of the Hale telescope. The large telescopes have proven useful in taking spectra of the optical counterparts of gamma-ray bursts, proving that they are very far away; and in analyzing the distances to faraway galaxies and in measuring the redshifts of their spectra, leading to the current cosmological models of the expansion of the universe and the tentative conclusion that the rate of expansion is accelerating. See also Cosmology; Hubble constant.

The 1990s saw the thorough use of the vantage points of space for astronomical observation, exemplified by NASA's series of Great Observatories. In 1991 the Compton Gamma-Ray Observatory was launched, and in the following years mapped about one gamma-ray burst per day in addition to many other objects and events. The Hubble Space Telescope was launched in 1990 to study the ultraviolet and visible parts of the spectrum. Its repair in 1993, with secondary mirrors compensating for a focusing problem with the main mirror, brought it to full working order, and a 1996 upgrade included an improved two-dimensional spectrograph and infrared capability. The Chandra X-Ray Observatory, launched in 1999, provides high-resolution x-ray images, and is the same size and scope as Hubble. It studies various types of celestial objects and processes, such as black holes of stellar and galactic sizes. The Space Infrared Telescope Facility, the fourth of this series of Great Observatories, was launched in 2004 and renamed the Spitzer Space Telescope. Smaller spacecraft have also made valuable contributions. See also Black hole; Infrared astronomy; X-ray astronomy; X-ray telescope.

The atmosphere blocks most of the electromagnetic spectrum from reaching the Earth's surface, leaving windows of transparency mostly in the optical and radio parts of the spectrum. Radio astronomers have made the most of their window of transparency with such telescopes as the 100-m (328-ft) fully steerable telescope outside Bonn, Germany; the 330-m (1083-ft) Arecibo dish in Puerto Rico, which has some limited tracking ability, the Very Large Array of radio telescopes in New Mexico, and the Very Long Baseline Array. The ozone layer and other constituents of the atmosphere block the shortest wavelengths from penetrating to the Earth's surface, so observations of gamma rays, x-rays, and most of the ultraviolet region require telescopes in space. See also Ozone; Radio astronomy; Radio telescope.

Much of astronomy involves breaking down the incoming celestial radiation into its component wavelengths, a process known as spectroscopy. Spectroscopic studies can reveal the temperature of an object, the identity and proportions of its chemical elements, and the velocities of its constituents toward and away from the Earth. Light from the Sun and other objects is sometimes polarized, and studies of such polarization can tell about the magnetic fields present or about scattering processes. See also Astronomical spectroscopy; Polarimetry.

The expansive definition of a telescope includes anything used in astronomy to observe the sky. Several neutrino telescopes have been used to detect neutrinos from the Sun and, in one instance, from a supernova. The pace of observation of secondary cosmic rays as well as the few primary cosmic rays that reach the Earth is increasing. A pair of interferometers are being built on Earth to attempt direct detection of such gravitational waves, which should result from such distant events as the merger of two neutron stars. See also Cosmic rays; Neutrino; Solar neutrinos.

Theoretical calculations of the nature of astronomical objects or processes are known as theoretical astrophysics. The availability of supercomputers, powerful and fast computers capable of handling large amounts of data, has led to three-dimensional simulations of, for example, the formation of large-scale structure in the early universe. Models of the oscillations detectable on the Sun's surface through long-time-series observations are used to improve understanding of the solar interior, a process known as helioseismology. See also Helioseismology; Simulation; Supercomputer; Universe.

Laboratory astrophysics involves the measurement of basic parameters that are used in calculations of physical or chemical processes relevant to astronomy, such as cross sections of atomic and molecular collisional excitation and ionization. See also Molecular structure and spectra.

For more information on astronomy, visit Britannica.com.

The science of astronomy (to which Rome contributed nothing) developed comparatively late in Greece and, being for the Greeks essentially a system for predicting the positions of the heavenly bodies, it covered only a part of what we understand by the term. In the classical period interest was confined to observations of the solstice and equinox and of the rising and setting of the most obvious stars and constellations, for the practical ends of navigation and establishing a calendar. The fifth-century Athenian astronomer Meton tried to correlate the lunar year with the solar year in a 19-year cycle (c.435 BC), but it seems that his system was not incorporated into the Athenian calendar. By the fifth century BC it was known to some individuals that the earth is a sphere and that the moon receives its light from the sun, and Anaxagoras understood the principles of eclipses. Not till the fourth century can we be sure that the Greeks identified the five planets known to the ancient world, Venus, Mercury, Mars, Jupiter, and Saturn (see EUDOXUS). In that century became crystallized the picture of the cosmos which was accepted by Aristotle (see 4 (iii)) and given final shape in the second century AD by Ptolemy, who was able to calculate the position of all the known heavenly bodies at a given moment, predict eclipses, and foretell the appearance and disappearances of the planets and fixed stars. At the centre of the cosmos was the earth, a globe suspended in space, whose weight had taken it to the centre. The earth was surrounded by a system of concentric spherical shells, rotating at various speeds and about various axes; on these were carried the moon, sun, planets, and fixed stars. The terrestrial atmosphere reached as far as the moon. The outermost sphere, that of the fixed stars, was composed of the rarest of all elements, the aether, and revolved daily about the earth. Ptolemy in his Almagest gave canonical form to this view of the cosmos, and his work remained uncorrected except in small details throughout all antiquity until Copernicus (1473–1543) and Galileo (1564–1642). For a heliocentric view of the universe see ARISTARCHUS (1). See further the names cited under MATHEMATICS.

Colonial Americans lacked instruments and libraries. They had difficulty communicating with each other and often relied on English correspondents for news of other colonialists. During the seventeenth century, European astronomy was focused on extending Isaac Newton's mathematical description of the solar system, and a few Americans contributed their observations to the Royal Society of London. American observations served European theories.

When Venus passed in front of the sun in 1761 and 1769, transits revealed the distance of the earth from the sun. John Winthrop, professor of mathematics and natural philosophy at Harvard, organized an expedition to Newfoundland to observe the first transit. The Massachusetts Assembly assigned a ship to transport Winthrop's group and Harvard permitted him to take college instruments, provided they were insured against loss or damage. The observations were sent to Europe for analysis.

Winthrop lectured his students that determination of the distance of the earth from the sun would result in a deeper insight into God's wonderful works. Enlightenment faith in the discernable regularity of the universe also encouraged the study of astronomy in early American colleges. In its appeal to the Pennsylvania Assembly for funds to observe the 1769 transit, the American Philosophical Society, founded in Philadelphia in 1743, cited a more utilitarian goal, "the Promotion of Astronomy and Navigation, and consequently of Trade and Commerce." In a period of increasing cultural nationalism, the society also wanted to win recognition for American achievements.

Many of America's astronomers were surveyors. The self-taught American astronomer David Rittenhouse made his living as a clockmaker, but he was also a surveyor. In 1767, he constructed in Philadelphia his famous orrery, or mechanical planetarium, which represented with great precision the motions of the planets around the sun. The Pennsylvania assembly paid for it. The onset of the Revolutionary War suspended hope to build an observatory.

With political independence came a desire for cultural independence. However, little public patronage was forthcoming for astronomy in the early national period. In 1825, President John Quincy Adams pointed out that Europe had 130 "lighthouses of the skies" but the United States none. Yet his request for funds for a national observatory was denied.

The American Academy of Arts and Sciences, founded in Boston in 1780 by John Adams, published astronomical observations by Nathaniel Bowditch, a self-educated Salem seaman. His The New American Practical Navigator (1800) became the most widely used nautical guide, and in 1811, he observed a solar eclipse to improve the determination of the longitude of Cambridge. The European scientific community applauded Bowditch's translation and commentary on Pierre Laplace's Mécanique céleste. The American Academy offered to pay for publication, but Bowditch waited until he could afford to publish it himself.

Through much of the nineteenth century, the United States was a nation in development. While some of their European brethren made observations and contributed to the advance of knowledge, American astronomers often struggled with more mundane problems, including writing textbooks, acquiring books and journals for libraries, and building, equipping, and financing observatories. Elias Loomis, a professor at Western Reserve College in Ohio, at the University of the City of New York, and at Yale University, published An Introduction to Practical Astronomy in 1855 and a A Treatise on Astronomy in 1876, both of which went through numerous editions. At both New York and Yale, Loomis arranged to receive publications from European observatories on an exchange basis. His will left funds to pay observers and publish their results.

College observatories consisted of a small building and telescope intended for the education of undergraduate students, but they could not pay researchers or provide funds for publication. The University of North Carolina built an observatory in 1831, which lasted several years. Observatories were constructed at Yale (1830s), at Williams College (1838), at Western Reserve College (1838), at the Philadelphia High School (1838), at West Point (1839), and at Georgetown (1843). In 1839, Harvard lured William Cranch Bond from his private observatory to supply his own instruments and work for no salary. The great comet of 1843 aroused public interest, which manifested itself in public support to construct and endow the Harvard College Observatory. Harvard ordered from the German firm Merz and Mahler a twin to the Russian Pulkova Observatory's fifteen-inch (lens diameter) telescope, then the largest in the world.

There were also public observatories: the Cincinnati Observatory, whose cornerstone John Quincy Adams laid in 1843, and the Dudley Observatory in Albany, built between 1852–1856. The tribulations of the Cincinnati Observatory illustrate the obstacles to practicing astronomy in mid-nineteenth-century America. Ormsby MacKnight Mitchel, a West Point graduate, moved to Cincinnati and became professor of mathematics, civil engineering, mechanics, and machinery at Cincinnati College. His public lectures on astronomy led to the founding of the Cincinnati Astronomical Society and the municipal Cincinnati Observatory, funded by public subscription. After five hours of teaching, Mitchel would supervise construction of the observatory in the afternoons. Cincinnati purchased an 11.25-inch telescope from Merz and Mahler, but Mitchel spent much of his time displaying the heavens to subscribers, from 4:00 to 10:00 P.M. daily. He tried publishing a journal to raise money for auxiliary instruments and for his salary, the observatory having no endowment for operating expenses, and did make money from a book on popular astronomy and from surveying a railroad route. His observations of singular phenomena, a kind of natural history of the heavens, fell short of a new professional emphasis on measurement and theory, requiring considerable mathematical competence. Economic forces discouraged sustained, structured research.

A few would-be professional astronomers received training and employment with the Coast Survey, established in 1807 in response to commercial interests of seaboard states. In legislation for the Coast Survey in 1832, Congress explicitly declared that it did not authorize construction or maintenance of a permanent astronomical observatory. A decade later, the Naval Observatory was created surreptitiously, as part of the Depot of Charts and Instruments. Not until 1866, however, would the observatory begin a program of fundamental research in astronomy. Meanwhile, the Nautical Almanac, located in Cambridge, Massachusetts, was established under the Naval Observatory budget in 1849. It reported directly to the secretary of the navy, provided training and employment for a few astronomers, and improved navigation and raised America's scientific standing with an annual astronomical almanac more accurate and theoretically advanced than the British Nautical Almanac. Simon New-comb, one of America's best-known scientists at the end of the century, got his start at the Nautical Almanac, and also worked at the Naval Observatory. He analyzed the motions of the moon and planets.

There were also a few private observatories in America. Lewis Rutherfurd, a wealthy New Yorker and trustee of Columbia College, had a nine-inch diameter telescope, and also a small transit instrument belonging to Columbia College at his observatory at Second Avenue and Eleventh Street. The Coast Survey used this observatory in 1848 to determine the longitude of New York. Rutherfurd was a pioneer in astronomical photography. Not until late in the century, though, would individual American fortunes fund the establishment and sustenance of large observatories with systematic programs of scientific investigation carried on by full-time, paid employees.

The second half of the nineteenth century saw advances in telescope production, especially by the Boston firm of Alvan Clark & Sons. Their metal tubes were stiffer yet lighter than wooden telescopes. Larger pieces of optical glass were now available, and the Clarks figured the lens for the world's largest refracting telescope on five occasions: an 18.5-inch lens for the University of Mississippi in 1860, a 26-inch lens for the Naval Observatory in 1873 (with which Asaph Hall discovered Mars's moons in 1877), a 30-inch lens for the Pulkova Observatory in 1883, a 36-inch lens for the Lick Observatory of the University of California in 1887, and a 40-inch lens for the Yerkes Observatory of the University of Chicago in 1897. James Lick, a California land speculator during the gold rush, and Charles Yerkes, a Chicago street car magnate, put up the funds for their eponymous observatories, under university auspices, and Boston investor Percival Lowell directed his own observatory. All three observatories were far removed from cities, and Lick's and Lowell's were on mountain peaks. With the largest telescopes in the best locations, American observatories now surpassed all others.

Growing interest in astrophysics and in distant stars and nebulae encouraged the development of new observatories with large steerable reflecting (light focused by a curved mirror) telescopes suitable for photography and auxiliary instruments for the analysis of starlight. George Ellery Hale founded the Astrophysical Journal in 1895, the American Astronomical and Astrophysical Society in 1899, the Mount Wilson Observatory in 1904, and the International Astronomical Union in 1918. Hale was an early prototype of the high-pressure, heavy-hardware, big-spending, team-organized scientific entrepreneur. In 1902, Andrew Carnegie, rich from innovations in the American steel industry, created the Carnegie Institution of Washington to encourage investigation, research, and discovery in biology, astronomy, and the earth sciences. Its ten million dollars were more than the total of endowed funds for research in all American universities combined. Hale left the Yerkes Observatory to build, with Carnegie money, the Mount Wilson Observatory on a mountain above Los Angeles. There George Willis Ritchey, who accompanied Hale from Yerkes, made the photographic reflecting telescope the basic instrument of astronomical research, constructing a 60-inch telescope in 1908 and a 100-inch telescope in 1919. They were the largest telescopes in the world and revolutionized the study of astronomy. Harlow Shapley found that the system of stars is a hundred times larger than previous estimates and that the sun is far from the center. Edwin Hubble showed that spiral nebulae are independent island universes beyond our galaxy and that the universe is expanding. Cosmology, previously limited to philosophical speculations, joined mainstream astronomy.

The Mount Wilson Observatory depended on its relationship with physicists at the nearby California Institute of Technology for its dominance of astrophysics during the first half of the twentieth century. A scientific education was fast becoming necessary for professional astronomers, as astrophysics came to predominate, and the concerns of professionals and amateurs diverged. As late as the 1870s and 1880s, the self-educated American astronomer Edward Emerson Barnard, an observaholic with indefatigable energy and ocular acuteness, could earn positions at the Lick and Yerkes observatories with visual observations of planetary details and discoveries of comets and moons. Already, however, he was an exception and an anachronism. Soon an advanced academic degree and considerable theoretical understanding were required of professional astronomers in America.

Supposedly, only men could withstand the rigors of observing the heavens all night in unheated telescope domes. Women were first employed to examine photo-graphs of stellar spectra and to catalog the spectra. Edward Pickering, director of the Harvard College Observatory in 1881 and an advocate of advanced study for women, was so exasperated with his male assistant's inefficiency that he declared even his cook could do a better job of copying and computing. Pickering hired her and she did do a better job, as did some twenty more females over the next several decades, recruited for their steadiness, adaptability, acuteness of vision, and willingness to work for low wages. In 1925, Cecilia Payne, a graduate student, determined the relative abundances of eighteen chemical elements found in stellar atmospheres. Her Ph.D. thesis has been lauded as the most brilliant written in astronomy. Her degree, however, was from Radcliffe College, before Harvard granted degrees to women, and in subsequent employment at Harvard she was initially budgeted as "equipment."

Radio astronomy began in America in 1933. Karl Jansky, a radio engineer with the Bell Telephone Company, detected electrical emissions from the center of our galaxy while studying sources of radio noise. Optical astronomers were not interested, nor were Jansky's practical-minded supervisors. Grote Reber, an ardent radio amateur obsessed with distance communication, was interested, and built for a few thousand dollars a 31.4-foot-diameter pointable radio antenna in his backyard in Wheaton, Illinois. In 1940, he reported the intensity of radio sources at different positions in the sky. Fundamental knowledge underlying radio astronomy techniques increased during World War II, especially with research on radar.

Advances in nuclear physics during the war made possible quantitative calculations of the formation of elements in a supposed primeval fireball. The Russian-American physicist George Gamow sought to explain the cosmic abundance of elements as the result of thermo-nuclear reactions in an early hot phase of an expanding universe, consisting of high-energy radiation. In 1963, unaware of Gamow's work, Arno Penzias and Robert Wilson at the Bell Telephone Laboratories detected radiation of cosmic origin. Meanwhile, Robert Dicke at Princeton University had independently thought of the cosmic background radiation and set a colleague to work calculating its strength. When Dicke learned in 1965 of Penzias and Wilson's measurement, he correctly interpreted it as Gamow's predicted radiation. A Nobel Prize went to Penzias and Wilson. Their discovery won general acceptance of the big bang theory and refuted the rival steady state theory.

World War II changed the relationship between science and the state. Radar, missiles, and the atomic bomb established state-sponsored and state-directed research and development. Furthermore, groups of scientists brought together in wartime proved effective. After the war, engineers and physicists with their instruments, techniques, training, and ways of operating moved into astronomy. Then came Sputnik in 1957, the world's first satellite. This Soviet triumph challenged American supremacy in military might and world opinion.

After Sputnik, the National Science Foundation supplied many millions of dollars for construction of the Kitt Peak National Observatory on a mountain near Tucson, Arizona. It is the largest collection of big telescopes in the Northern Hemisphere. Seventeen universities came together in AURA, the Association of Universities for Re-search in Astronomy, to manage the observatory.

Another response to Sputnik was the creation of the National Aeronautics and Space Administration (NASA). Among its accomplishments are automated observatories launched into space, including the Hubble Space Telescope. Its primary mirror is eight feet in diameter. Including recording instruments and guidance system, the telescope weighs twelve tons. It has been called the eighth wonder of the world, and critics say it should be, given its cost of 1.5 billion dollars! The telescope is as much a political and managerial achievement as a technological one. Approval for a large space telescope was won in a political struggle lasting from 1974 to 1977, but not until 1990 were a plethora of problems finally overcome and the telescope launched into space, only to discover that an error had occurred in the shaping of the primary mirror. One newspaper reported "Pix Nixed as Hubble Sees Double." The addition of a corrective mirror solved the problem.

NASA also funds X-ray astronomy. Captured German rockets provided the first proof of X-rays from the sun. Astronomers did not expect to find X-ray sources and were skeptical that brief and expensive rocket-borne experiments were worthwhile. NASA, however, had more money than there were imaginative scientists to spend it, and the military, even more. One imaginative and eager scientist was the Italian-born Riccardo Giacconi, who in 1960, funded by the Air Force Cambridge Research Laboratories, discovered a cosmic X-ray source, and in 1963, detected a second. NASA adjudicates questions of scientific priority and supplies money for space observatories; industry helps build them; universities or consortiums of universities design and operate them and analyze the data. NASA then funded a rocket survey program and a small satellite for X-ray astronomy and in 1978 the Einstein X-ray telescope. Unlike the relatively quiescent universe seen by earth-bound astronomers, the universe revealed to engineers and physicists observing from satellites is violently energetic.

Major changes have occurred in both the size and scope of American astronomy over the centuries, but never more rapidly nor more dramatically than at the beginning of the Space Age. There were some five hundred American astronomers in 1962 and three times that many a decade later. Only four worked on X-rays in 1962 compared to over forty times that many in 1972. Over eighty percent of them were migrants from experimental physics, with expertise in designing and building instruments to detect high-energy particles.

Astronomers now realize that important cosmological features can be explained as consequences of new theories of particle physics, and particle physics increasingly drives cosmology. Conversely, particle physicists, having exhausted the limits of particle accelerators and public funding for yet larger instruments, turn to cosmology for information regarding the behavior of matter under extreme conditions, such as those prevailing in the early universe.

The spectacular rise of American astronomy roughly parallels the remarkable evolution of the nation, itself, from British colonies to world super power. Once limited to visual observations and determining positions, astronomy now includes cosmology, the study of the structure and evolution of the universe, and analysis of the physical and chemical composition of the universe and its components. Once peripheral, now American astronomers, men and women, formally educated in a variety of fields, working in large teams, on systematic long-term projects, and enjoying government patronage, lead world advances in instrumentation, observation, and theory.

Bibliography

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———. Yerkes Observatory 1892–1950: The Birth, Near Death, and Resurrection of a Scientific Research Institution. Chicago: University of Chicago, 1997.

———. Pauper and Prince: Ritchey, Hale, & Big American Telescopes. Tucson: University of Arizona Press, 1993.

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Warner, Deborah Jean. Alvan Clark & Sons: Artists in Optics. Richmond, Va.: Willmann-Bell, 1995.

—Norriss Hetherington

The movement of the stars and planets has fascinated humans for thousands of years. For the vast majority of ancient astronomers, the stars seemed to be equally distant from Earth in what was an Earth-centered (or "geocentric") cosmos. The ancient Greeks observed that the stars revolved westward around the north celestial pole every twenty-three hours and fifty-six minutes, thus constituting a kind of objective clock. Most envisaged these revolving stars to be located on a sphere, composing an incorruptible, celestial orb that could easily be contrasted with the world of change, generation, and corruption on Earth (to which irregular meteorological phenomena such as comets were also understood to belong). Against the backdrop of this outer sphere, the Sun was seen to move along a path termed the ecliptic, while the planets moved within eight degrees of this path. These usually moved eastward, though occasionally they moved in the opposite direction, thus exhibiting retrograde motion. Many Greek astronomers suggested that the Sun could be placed on the equator of an inner sphere that revolved once a year, hence constituting a second sphere in addition to the stellar orb. Following the theories of the fourth-century-B.C.E. scholar Eudoxus, astronomers and natural philosophers became increasingly committed to the idea that the motions of each planet could be accounted for by means of their own specific, homocentric spheres.

Defending an Earth-Centered Universe

The retrograde motion of the planets offended against the apparent simplicity of heavenly motions, as well as the dictum that their orbits were circular. From the third century B.C.E., astronomers began to conceive of planets rotating in small circles (epicycles) around a point that moved on a carrier orbit (or deferent). This accounted for the apparent motions of the planets, including their retrograde motion, although others followed Aristotle in positing the existence of homocentric spheres. After Ptolemy's composition of the Almagest in the second century C.E., the movements of the planets could be accurately represented by means of techniques involving the use of epicycles, deferents, eccentrics (whereby planetary motion is conceived as circular with respect to a point displaced from Earth), and equants (a device that posits a constant angular rate of rotation with respect to a point displaced from Earth). These approaches, and arguments over the reality of the celestial orbs, continued to be refined in the following centuries by first Muslim and then Christian astronomers. The geocentric cosmos was readily appropriated by Christians, many of whom believed that God existed outside the stellar sphere and (as evidenced by Dante's fourteenth-century poem Divine Comedy) that angels turned the epicycles and spheres in the intervening spaces.

Astronomy had a primarily practical function, and by the thirteenth century it played a central role in determining the dates of religious festivals. Ptolemy's mathematically sophisticated Almagest, with various commentaries, formed the basis for advanced astronomy in university curricula from this period onward. In the late fifteenth century, the arrival of the printing press coincided with the transmission to western Europe of a number of important Greek mathematical and astronomical manuscripts. These became a new focal point for doing astronomy, as ancient texts could be seen and assessed in their original form (as opposed to being translated into Latin from Syriac or Arabic). Georg von Peuerbach (1423–1461) and his pupil Regiomontanus produced important works that formed the basis of astronomical training for the next century.

Astronomy was equally significant for providing data from which astrological predictions could be made (judicial astrology). The effects that heavenly actions had on earthly events, and the manner in which they were carried out, constituted a problem for most human societies. Astrology was a major part of both daily life and intellectual culture, and was deeply implicated in politics, medicine, and agriculture. However, its use was increasingly questioned by elites in the late sixteenth century; both Catholics and Protestants argued that the notion that the relations between stars and planets influenced or governed human behavior detracted from the primary Christian belief in free will. Nevertheless, this argument was often expressed as a condemnation of bad astrology, the implication being that astrology was a credible art or science that could be reformed. However, although it was still taken seriously by Johannes Kepler at the beginning of the seventeenth century, it was almost universally despised by social elites a hundred years later.

In Aristotle's De Caelo, heavenly bodies were treated as physical objects and thus as a branch of natural philosophy, or physics. Astronomy thus enjoyed a somewhat bifurcated position within the university system, taught partly in tandem with medicine and astrology, and partly as a higher-level, more technically difficult mathematical enterprise. This mirrored the scholarly distinction between the treatment of the physical reality of the supposedly "crystalline" spheres and such ideas as the epicycles, the actual existence of which was debatable. When Nicolaus Copernicus (1473–1543) published his heliocentric (Sun-centered) De Revolutionibus Orbium Coelestium (On the revolutions of the heavenly orbs) of 1543, a number of astronomers believed that his system could be treated as another convenient set of devices for explaining the celestial phenomena without any commitment to the reality of the general cosmology and physics that was prominent in the first part of the book. Indeed, the Lutheran theologian Andreas Osiander (1498–1552) reassured readers in an anonymous preface to the work that Copernicus had merely devised "hypotheses" that facilitated the calculation of celestial positions.

The Copernican Revolution

However, Copernicus had indeed upset the standard disciplinary division in universities, and he implied that a mathematical astronomer was entitled to speak about the physical nature of things—the traditional preserve of the more prestigious role of the natural philosopher. Not only did he offer the first modern heliocentric system (although he appealed to the authority of ancient heliocentric systems such as that of Pythagoras), but he asserted that the demonstrations in his work could only be understood by mathematically competent astronomers. He condemned the uncertainty of the calendar, and also of the inability of astronomers to determine the precise motions of the heavenly bodies. He lambasted the inconsistent use of homocentric circles and epicycles, and argued that the systems produced by recent astronomers lacked aesthetic credibility. Instead, with all the different techniques and philosophies in play, they had produced a sort of astronomical "monster."

Although he retained epicycles, Copernicus dispensed with the equant and showed that retrograde motion could be explained by means of a heliocentric system. Indeed, contra Osiander, he could hardly have been more assertive about the truth of his Sun-centered cosmos. He proffered tentative physical explanations of why objects on a revolving earth would not be ejected into the heavens, and argued that the stellar sphere had to be an enormous distance further away than that accepted by traditional astronomers. This meant that his inability to detect stellar parallax (the feature by which the apparent position of a star would vary depending on the time of year, since an observer on Earth would view it from a different position) would not be an argument against his system. Ominously, he also suggested that biblical passages that appeared to support geocentricity were the result of the author "accommodating" his discourse to the capacities of ordinary people. Momentous as it now seems, Copernicus's system had only a handful of adherents in the sixteenth century, although Erasmus Reinhold's Prutenic Tables of 1551, which were based on methods pioneered by Copernicus, were used to produce the Gregorian calendar (decreed by Pope Gregory XIII) in 1582.

In the last three decades of the sixteenth century, the Danish astronomer Tycho Brahe (1546–1601) was single-handedly responsible for two major initiatives in astronomy. First, he devised a "geoheliocentric" system that, for the half-century before Copenicus's system was broadly accepted, provided the major alternative to a geocentric cosmos. In this, the Sun again revolved around the Earth, with the five planets (not including the Moon) revolving around the Sun; since the Martian and solar orbits intersected, this system could not accommodate the reality of the spheres. Tycho rejected a heliocentric system, partly because he could not abide the massive distances required by Copernicus's system, partly because he could not find stellar parallax, and partly because he was strongly opposed to heliocentrism on scriptural grounds.

Most important, Tycho made a series of naked-eye observations, conducted from 1574 as part of a monumental observation program at his observatory (Uraniborg) on the island of Hven. He designed and made new instruments that were as much as ten times more accurate than those of his predecessors. He also conceived of a means of using each of his devices to cross-check results obtained from other instruments in his collection. His instruments were less cumbersome than previous examples and he introduced new techniques for more precisely dividing them into minutes and seconds of a degree. He turned himself and his workplace—which had its own printing press—into the hub of an extensive network of correspondents, and he used this and personal contacts to train a number of the best astronomers of the next generation. Two of his measurements, linked to exceptional celestial events in the 1570s, demonstrated the precision of his observations. First, he determined the extreme distance from Earth of the terrifying supernova of 1572, and second, he showed for the first time that the comet of 1577 existed beyond the lunar sphere and hence was technically part of the heavens. Independent of any implications of De Revolutionibus, both seemed to suggest that the celestial sphere could no longer be considered immutable.

As Tycho's work came to a close, Johannes Kepler (1571–1630) burst on to the stage with his heliocentric Mysterium Cosmographicum (The secret of the universe) of 1596. Kepler was trained at Tübingen by the pro-Copernican Michael Mästlin (1550–1631), and throughout his adult life remained committed to a heliocentric system. In the Mysterium he famously attempted to show that the distances between the planets could be represented by nested regular solids, although this ultimately failed to fit the astronomical data produced by Tycho and his colleagues. At the end of 1600 Kepler traveled to Prague to work with Tycho, who had only recently been appointed imperial mathematician to Rudolf II. When Tycho died within the year, Kepler gained access to his data concerning the orbit of Mars. Tycho had observed the Martian orbit with astonishing accuracy, finding a discrepancy between the observed orbit and an ideal circular trajectory of eight feet that could not be explained by instrument error, and Kepler sought strenuously over the next few years to provide a harmonious and geometrically satisfying orbit to fit these observations. After many different shapes had been tried, he decided that Mars and the other planets traveled in ellipses, one of whose foci was the Sun, and that each planet swept out equal areas in equal times (considering the area to be drawn out by a line linking the planet to the Sun).

Kepler published his first two laws of planetary motion in his Astronomia Nova (New astronomy) of 1609, although they are merely a handful of many mathematical relations that he posed for planetary orbits. In this work he also appealed to the magnetic philosophy devised by the English physicist William Gilbert in his De Magnete (On the magnet) of 1600, in order to explain the physical causes of heavenly motion. He suggested that the planets, which all traveled in the same direction and (virtually) in the same plane, were controlled by a motive force emanating from the Sun. His third and final law, first enunciated in his Harmonice Mundi (Harmony of the world) of 1619, is more complicated and states that the square of the mean orbital period of a planet is proportional to the cube of its mean distance from the Sun. Kepler combined a Platonic-Pythagorean concern for the reality of harmonies and ratios with a magnetic physics, but from a disciplinary point of view he was explicitly asserting the right of an astronomer such as himself to produce a "celestial physics" that gave the true (non-Aristotelian) causes of heavenly motion. In a work on astrology of 1601, he also attempted to uncover the physical causes underlying the influence that planetary conjunctions had on earthly activities.

The Scientific Revolution

In 1609, the same year that Kepler published his first two laws, Galileo Galilei (1564–1642) used a combination of lenses to look at the heavens, having heard of a similar invention that had been introduced the year before. Within weeks, he had deduced that the Moon had mountains and valleys (and was thus not perfectly smooth), had noted that the Milky Way was actually composed of numerous stars, and proposed that Jupiter possessed four satellites that revolved around it as the Moon revolved around Earth. For his extraordinary discoveries, which appeared in Sidereus Nuncius (Starry messenger) of 1610, he was rewarded with the position of court philosopher to his onetime pupil, Cosimo de' Medici (Cosimo II), grand duke of Tuscany. Galileo was now a court intellectual to rival Johannes Kepler, who had succeeded Tycho Brahe as imperial mathematician to Rudolf II. Given the courtly locations of both Kepler and Galileo, twentieth- and twenty-first-century historians have pointed to the prevalence of nonuniversity locations as the most important settings for innovative astronomy. Kepler's friendly stance toward Galileo (he wrote a book praising Galileo's discoveries within months of the work appearing) ensured that there was minimal animosity between them, although this may have been helped by Galileo's apparent complete ignorance of Kepler's own discoveries.

In the next two and a half decades, adherence to the Copernican system met with determined opposition and, notoriously, René Descartes felt obliged to suppress his not sufficiently anti-Copernican Le monde (The world) in 1633 soon after Galileo's pro-Copernican Dialogue had been condemned by the Roman Inquisition in July. However, the vast majority of astronomers and natural philosophers were avowed Copernicans by the middle of the seventeenth century, and even those, such as the Jesuits, who were slow to accept the Copernican worldview, made substantial contributions to astronomy. Indeed, Galileo's successes can be overstated as contributions to the demise of the notion of a perfect celestial sphere, for previous events and observations had begun to shake confidence in heavenly incorrigibility. Of all his discoveries, only his sightings of the phases of Venus provided overt support for Copernicus's system, although Tycho's system could also account for them. Nevertheless, Galileo's visually striking discoveries—and their implications—were easily understood by a new audience of scholars and gentlemen, and they had a dramatic impact on writers and poets such as John Donne (1572–1631).

It is important to note, as Allan Chapman has properly argued, that the great theoretical advances in cosmology achieved by Kepler, Isaac Newton, and others were facilitated and indeed made possible by improvements in angular astronomical measurement and not by superior visual acuity. Tycho's advances in instrument design and observational accuracy were made possible by the cadre of excellent craftsmen he had at his disposal, and similarly, John Flamsteed, the first British astronomer royal, whose observations were to prove crucial for Newton's enunciation of universal gravitation, had innovative and highly skilled instrument makers working with him. Only with patient astronomy of this sort could precise measurements be made of celestial magnitudes such as distance and size.

In the 1660s telescopic sights were added to quadrants and a zenith sector in the attempt by the French to measure the length of a degree of meridian in France. In Restoration England a number of episodes occurred that acted as a spur to creating an alliance between accurate measurement and theoretical innovation. The Royal Society of London was founded in 1660, followed by the Royal Greenwich Observatory—founded to aid navigation and the determination of longitude by improving astronomy—in 1675. The Observatory was badly stocked with instruments at first, but gradually, with some private support and with the help of occasionally brilliant suggestions from Robert Hooke (1635–1703) for automating observations, Flamsteed was able to build up a stock of the best instruments then available. By the 1690s he had a degree clock that allowed star positions to be measured with extraordinary accuracy, and the ten-to-twelveseconds error of his mural arc (that is, a large quadrant set on a wall) was a sixfold improvement on the accuracy of Tycho's instruments. Flamsteed used telescopic sights on his instruments but also a filar micrometer, that is, a system of thin wires, minutely movable by means of a carefully graduated screw, that could be placed inside a telescope to finesse its accuracy.

Isaac Newton (1642–1727) developed an early interest in astronomy and became famous initially because of his development of a reflecting telescope in the late 1660s. Flamsteed's data was crucial for Newton in the winter of 1684–1685, when the latter was trying to determine what mutual influence Jupiter and Saturn might have on each other, and again in late 1685, when Newton wanted three items of data (accurate to a minute) on the path of the Great Comet of 1680–1681. This data would constitute crucial evidence for the cosmological system that Newton published in his momentous Principia Mathematica (The mathematical principles of natural philosophy) of 1687, for he could now analyze observed deviations from perfect elliptical orbits by means of his concept of universal gravitation. Perhaps of equal significance were the observations Flamsteed put his way in 1694–1695 when Newton had another go at the Moon. This ultimately unprofitable endeavor was part of an effort to solve the (insoluble) three-body problem of the mutual interactions of Sun, Moon, and Earth, all of which Newton later described as the most difficult science he ever did. The pair fell out irreconcilably soon after this, and Newton behaved abominably toward the astronomer royal, practically stealing Flamsteed's laboriously crafted star catalog by claiming it as the property of the state. Not the least of Newton's actions was to downgrade and even efface (in his Principia) the contributions made by Flamsteed, who had generously provided the observations that allowed Newton to corroborate and then rework his supreme theory. Whatever his personal dealings with others, Newton's theory provided the basic theory of the heavens that we now take to be true, and his achievements included the recognition that some comets travel in periodic elliptical orbits.

By the early eighteenth century, the London instrument-making trade was widely held to produce the highest quality instruments; Pierre-Louis Moreau de Maupertuis (1698–1759), for example, took a zenith sector and clock constructed by the outstanding London instrument maker George Graham (1673–1751) to Lapland in 1736–1737. It was this expedition that went furthest in determining the shape of Earth, confirming Newton's calculation that it was an oblate spheroid (flattened at the poles). In England, Graham and others made instruments for the astronomers royal who followed Flamsteed, namely Edmond Halley (in 1720) and James Bradley (in 1742). Bradley, who discovered stellar aberration in 1727 and who confirmed Newton's analysis of the extent of the nutation of Earth's axis in the 1740s, combined access to the best instruments of the day with an obsession for accuracy. By the middle of the eighteenth century, measurements were confined to the meridian, and, among other activities, experiments were being undertaken to better ascertain longitude and latitude—an activity seen by the British, the French, and many other naval powers as essential for improving navigation. With massively expensive instrumentation that only large institutions could afford, astronomy had changed beyond all recognition from the medieval period. Religious and other value systems no longer placed barriers on believing in and publishing particular accounts of the cosmos, and all serious intellectuals were heliocentrists.

Bibliography

Primary Sources

Copernicus, Nicolaus. On the Revolutions of the Heavenly Spheres. Translated by A. M. Duncan. Newton Abbott, U.K., and New York, 1976.

Descartes, René. The World and Other Writings. Translated and edited by Stephen Gaukroger. Cambridge, U.K., and New York, 1998.

Galilei, Galileo. Sidereus Nuncius; or, The Sidereal Messenger. Translated by Albert Van Helden. Chicago and London, 1989.

Kepler, Johannes. Mysterium Cosmographicum: The Secret of the Universe. Translated by A. M. Duncan. New York, 1981.

——. New Astronomy. Translated by William H. Donahue. Cambridge, U.K., and New York, 1992.

Secondary Sources

Chapman, Allan. Dividing the Circle: The Development of Critical Angular Measurement in Astronomy, 1500–1850. 2nd ed. Chichester, U.K., and New York, 1995.

Donahue, William H. The Dissolution of the Celestial Spheres. New York, 1981.

Dreyer, J. L. E. A History of Astronomy from Thales to Kepler. Rev. ed., with foreword by W. H. Stahl. New York, 1953.

Jardine, Nicholas. The Birth of History and Philosophy of Science: Kepler's A Defence of Tycho against Ursus, with Essays on its Provenance and Significance. Cambridge, U.K., and New York, 1984.

King, Henry C. The History of the Telescope. New York, 1979.

Kuhn, Thomas S. The Copernican Revolution: Planetary Astronomy in the Development of Western Thought. Cambridge, Mass., and London, 1957.

Newman, William R., and Anthony Grafton, eds. Secrets of Nature: Astrology and Alchemy in Early Modern Europe. Cambridge, Mass., and London, 2001.

Schechner Genuth, Sara. Comets, Popular Culture, and the Birth of Modern Cosmology. Princeton, 1997.

Stephenson, Bruce. Kepler's Physical Astronomy. Princeton, 1994.

Thoren, Victor E., with contributions by John R. Christianson. The Lord of Uraniborg: A Biography of Tycho Brahe. Cambridge, U.K., and New York, 1990.

—ROB ILIFFE

The science that deals with the universe beyond the Earth. It describes the nature, position, and motion of the stars, planets, and other objects in the skies, and their relation to the Earth.

Quotes:

"Astronomy is perhaps the science whose discoveries owe least to chance, in which human understanding appears in its whole magnitude, and through which man can best learn how small he is." - Georg C. Lichtenberg

"Adam inquires concerning celestial motions, is doubtfully answered, and exhorted to search rather things more worthy of knowledge." - John Milton

"It is clear to everyone that astronomy at all events compels the soul to look upwards, and draws it from the things of this world to the other." - Plato

"These earthly godfathers of Heaven's lights, that give a name to every fixed star, have no more profit of their shining nights than those that walk and know not what they are." - William Shakespeare

"I try to forget what happiness was, and when that don't work, I study the stars." - Derek Walcott

"When I, sitting, heard the astronomer, where he lectured with such applause in the lecture room, how soon, unaccountable, I became tired and sick; Till rising and gliding out, I wandered off by myself, in the mystical moist night-air, and from time to time, looked up in perfect silence at the stars." - Walt Whitman

See more famous quotes about Astronomy

For other uses, see Astronomy (disambiguation).

A giant Hubble mosaic of the Crab Nebula, a supernova remnant

Astronomy (from the Greek words astron (ἄστρον), "star", and nomos (νόμος), "law") is the scientific study of celestial objects (such as stars, planets, comets, and galaxies) and phenomena that originate outside the Earth's atmosphere (such as the cosmic background radiation). It is concerned with the evolution, physics, chemistry, meteorology, and motion of celestial objects, as well as the formation and development of the universe.

Astronomy is one of the oldest sciences. Astronomers of early civilizations performed methodical observations of the night sky, and astronomical artifacts have been found from much earlier periods. However, the invention of the telescope was required before astronomy was able to develop into a modern science. Historically, astronomy has included disciplines as diverse as astrometry, celestial navigation, observational astronomy, the making of calendars, and even astrology, but professional astronomy is nowadays often considered to be synonymous with astrophysics. Since the 20th century, the field of professional astronomy split into observational and theoretical branches. Observational astronomy is focused on acquiring and analyzing data, mainly using basic principles of physics. Theoretical astronomy is oriented towards the development of computer or analytical models to describe astronomical objects and phenomena. The two fields complement each other, with theoretical astronomy seeking to explain the observational results, and observations being used to confirm theoretical results.

Amateur astronomers have contributed to many important astronomical discoveries, and astronomy is one of the few sciences where amateurs can still play an active role, especially in the discovery and observation of transient phenomena.

Old or even ancient astronomy is not to be confused with astrology, the belief system which claims that human affairs are correlated with the positions of celestial objects. Although the two fields share a common origin and a part of their methods (namely, the use of ephemerides), they are distinct.^[1]

2009 has been declared by the UN to be the International Year of Astronomy 2009 (IYA2009). The focus is on enhancing the public’s understanding and engagement with astronomy.

Contents 1 Lexicology 1.1 Use of terms "astronomy" and "astrophysics" 2 History 2.1 Scientific revolution 3 Observational astronomy 3.1 Radio astronomy 3.2 Infrared astronomy 3.3 Optical astronomy 3.4 Ultraviolet astronomy 3.5 X-ray astronomy 3.6 Gamma-ray astronomy 3.7 Fields of observational astronomy not based on the electromagnetic spectrum 3.8 Astrometry and celestial mechanics 4 Theoretical astronomy 5 Specific subfields of astronomy 5.1 Solar astronomy 5.2 Planetary science 5.3 Stellar astronomy 5.4 Galactic astronomy 5.5 Extragalactic astronomy 5.6 Cosmology 6 Interdisciplinary studies 7 Amateur astronomy 8 Major questions in astronomy 9 The International Year of Astronomy 2009 10 See also 11 References 12 External links

Lexicology

The word astronomy literally means "law of the stars" (or "culture of the stars" depending on the translation) and is derived from the Greek αστρονομία, astronomia, from the words άστρον (astron, "star") and νόμος (nomos, "laws or cultures").

Use of terms "astronomy" and "astrophysics"

Generally, either the term "astronomy" or "astrophysics" may be used to refer to this subject.^[2][3][4] Based on strict dictionary definitions, "astronomy" refers to "the study of objects and matter outside the Earth's atmosphere and of their physical and chemical properties"^[5] and "astrophysics" refers to the branch of astronomy dealing with "the behavior, physical properties, and dynamic processes of celestial objects and phenomena".^[6] In some cases, as in the introduction of the introductory textbook The Physical Universe by Frank Shu, "astronomy" may be used to describe the qualitative study of the subject, whereas "astrophysics" is used to describe the physics-oriented version of the subject.^[7] However, since most modern astronomical research deals with subjects related to physics, modern astronomy could actually be called astrophysics.^[2] Various departments that research this subject may use "astronomy" and "astrophysics", partly depending on whether the department is historically affiliated with a physics department,^[3] and many professional astronomers actually have physics degrees.^[4] One of the leading scientific journals in the field is named Astronomy and Astrophysics.

History

Main article: History of astronomy

Further information: Archaeoastronomy

A celestial map from the 17th century, by the Dutch cartographer Frederik de Wit.

In early times, astronomy only comprised the observation and predictions of the motions of objects visible to the naked eye. In some locations, such as Stonehenge, early cultures assembled massive artifacts that likely had some astronomical purpose. In addition to their ceremonial uses, these observatories could be employed to determine the seasons, an important factor in knowing when to plant crops, as well as in understanding the length of the year.^[8]

Before tools such as the telescope were invented early study of the stars had to be conducted from the only vantage points available, namely tall buildings and high ground using the bare eye. As civilizations developed, most notably in Mesopotamia, Greece, Egypt, Persia, Maya, India, China, Nubia^[9] and the Islamic world, astronomical observatories were assembled, and ideas on the nature of the universe began to be explored. Most of early astronomy actually consisted of mapping the positions of the stars and planets, a science now referred to as astrometry. From these observations, early ideas about the motions of the planets were formed, and the nature of the Sun, Moon and the Earth in the universe were explored philosophically. The Earth was believed to be the center of the universe with the Sun, the Moon and the stars rotating around it. This is known as the geocentric model of the universe.

A number of notable astronomical discoveries were made prior to the application of the telescope. For example, the obliquity of the ecliptic was estimated as early as 1000 BC by Chinese astronomers. The Chaldeans discovered that lunar eclipses recurred in a repeating cycle known as a saros.^[10] In the 2nd century BC, the size and distance of the Moon were estimated by Hipparchus^[11] and later Arabic astronomers. The Andromeda Galaxy, the nearest galaxy to the Milky Way, was discovered in 964 by the Persian astronomer Azophi and first descrbed in his Book of Fixed Stars.^[12] The SN 1006 supernova, the brightest apparent magnitude stellar event in recorded history, was observed by the Egyptian Arabic astronomer Ali ibn Ridwan and the Chinese astronomers in 1006.

The earliest known astronomical device was the Antikythera mechanism, an ancient Greek device for calculating the movements of planets, that dates from about 150-80 BC, and was the earliest ancestor of an astronomical analog computer. Similar astronomical analog computing devices were later constructed by Arabic astronomers and then European astronomers.

During the Middle Ages, observational astronomy was mostly stagnant in medieval Europe, at least until the 13th century. However, astronomy flourished in the Islamic world and other parts of the world. Some of the prominent Arabic astronomers who made significant contributions to the science include Al-Battani, Thebit, Azophi, Albumasar, Biruni, Arzachel, the Maragha school, Qushji, Al-Birjandi, Taqi al-Din, and others. Astronomers during that time introduced many Arabic names now used for individual stars.^[13][14] It is also believed that the ruins at Great Zimbabwe and Timbuktu^[15] may have housed an astronomical observatory.^[16] Europeans had previously believed that there had been no astronomical observation in pre-colonial Middle Ages sub-Saharan Africa but modern discoveries show otherwise.^[17][18][19]

Scientific revolution

Galileo's sketches and observations of the Moon revealed that the surface was mountainous

During the Renaissance, Nicolaus Copernicus proposed a heliocentric model of the solar system. His work was defended, expanded upon, and corrected by Galileo Galilei and Johannes Kepler. Galileo innovated by using telescopes to enhance his observations.

Kepler was the first to devise a system that described correctly the details of the motion of the planets with the Sun at the center. However, Kepler did not succeed in formulating a theory behind the laws he wrote down. It was left to Newton's invention of celestial dynamics and his law of gravitation to finally explain the motions of the planets. Newton also developed the reflecting telescope.

Further discoveries paralleled the improvements in the size and quality of the telescope. More extensive star catalogues were produced by Lacaille. The astronomer William Herschel made a detailed catalog of nebulosity and clusters, and in 1781 discovered the planet Uranus, the first new planet found. The distance to a star was first announced in 1838 when the parallax of 61 Cygni was measured by Friedrich Bessel.

During the nineteenth century, attention to the three body problem by Euler, Clairaut, and D'Alembert led to more accurate predictions about the motions of the Moon and planets. This work was further refined by Lagrange and Laplace, allowing the masses of the planets and moons to be estimated from their perturbations.

Significant advances in astronomy came about with the introduction of new technology, including the spectroscope and photography. Fraunhofer discovered about 600 bands in the spectrum of the Sun in 1814-15, which, in 1859, Kirchhoff ascribed to the presence of different elements. Stars were proven to be similar to the Earth's own Sun, but with a wide range of temperatures, masses, and sizes.^[13]

The existence of the Earth's galaxy, the Milky Way, as a separate group of stars, was only proved in the 20th century, along with the existence of "external" galaxies, and soon after, the expansion of the universe, seen in the recession of most galaxies from us. Modern astronomy has also discovered many exotic objects such as quasars, pulsars, blazars, and radio galaxies, and has used these observations to develop physical theories which describe some of these objects in terms of equally exotic objects such as black holes and neutron stars. Physical cosmology made huge advances during the 20th century, with the model of the Big Bang heavily supported by the evidence provided by astronomy and physics, such as the cosmic microwave background radiation, Hubble's law, and cosmological abundances of elements.

Observational astronomy

The Very Large Array in New Mexico, an example of a radio telescope.

Main article: Observational astronomy

In astronomy, information is mainly received from the detection and analysis of visible light or other regions of the electromagnetic radiation.^[20] Observational astronomy may be divided according to the observed region of the electromagnetic spectrum. Some parts of the spectrum can be observed from the Earth's surface, while other parts are only observable from either high altitudes or space. Specific information on these subfields is given below.

Radio astronomy

Main article: Radio astronomy

Radio astronomy studies radiation with wavelengths greater than approximately one millimeter.^[21] Radio astronomy is different from most other forms of observational astronomy in that the observed radio waves can be treated as waves rather than as discrete photons. Hence, it is relatively easier to measure both the amplitude and phase of radio waves, whereas this is not as easily done at shorter wavelengths.^[21]

Though some radio waves are produced by astronomical objects in the form of thermal emission, most of the radio emission that is observed from Earth is seen in the form of synchrotron radiation, which is produced when electrons oscillate around magnetic fields.^[21] Additionally, a number of spectral lines produced by interstellar gas, notably the hydrogen spectral line at 21 cm, are observable at radio wavelengths.^[7][21]

A wide variety of objects are observable at radio wavelengths, including supernovae, interstellar gas, pulsars, and active galactic nuclei.^[7][21]

Infrared astronomy

Main article: Infrared astronomy

Infrared astronomy deals with the detection and analysis of infrared radiation (wavelengths longer than red light). Except at wavelengths close to visible light, infrared radiation is heavily absorbed by the atmosphere, and the atmosphere produces significant infrared emission. Consequently, infrared observatories have to be located in high, dry places or in space. The infrared spectrum is useful for studying objects that are too cold to radiate visible light, such as planets and circumstellar disks. Longer infrared wavelengths can also penetrate clouds of dust that blocks visible light, allowing observation of young stars in molecular clouds and the cores of galaxies.^[22] Some molecules radiate strongly in the infrared, and this can be used to study chemistry in space, as well as detecting water in comets.^[23]

Optical astronomy

The Subaru Telescope (left) and Keck Observatory (center) on Mauna Kea, both examples of an observatory that operates at near-infrared and visible wavelengths. The NASA Infrared Telescope Facility (right) is an example of a telescope that operates only at near-infrared wavelengths.

Main article: Optical astronomy

Historically, optical astronomy, also called visible light astronomy, is the oldest form of astronomy.^[24] Optical images were originally drawn by hand. In the late nineteenth century and most of the twentieth century, images were made using photographic equipment. Modern images are made using digital detectors, particularly detectors using charge-coupled devices (CCDs). Although visible light itself extends from approximately 4000 Å to 7000 Å (400 nm to 700 nm),^[24] the same equipment used at these wavelengths is also used to observe some near-ultraviolet and near-infrared radiation.

Ultraviolet astronomy

Main article: Ultraviolet astronomy

Ultraviolet astronomy is generally used to refer to observations at ultraviolet wavelengths between approximately 100 and 3200 Å (10 to 320 nm).^[21] Light at these wavelengths is absorbed by the Earth's atmosphere, so observations at these wavelengths must be performed from the upper atmosphere or from space. Ultraviolet astronomy is best suited to the study of thermal radiation and spectral emission lines from hot blue stars (OB stars) that are very bright in this wave band. This includes the blue stars in other galaxies, which have been the targets of several ultraviolet surveys. Other objects commonly observed in ultraviolet light include planetary nebulae, supernova remnants, and active galactic nuclei.^[21] However, ultraviolet light is easily absorbed by interstellar dust, and measurement of the ultraviolet light from objects need to be corrected for extinction.^[21]

X-ray astronomy

Main article: X-ray astronomy

X-ray astronomy is the study of astronomical objects at X-ray wavelengths. Typically, objects emit X-ray radiation as synchrotron emission (produced by electrons oscillating around magnetic field lines), thermal emission from thin gases (called bremsstrahlung radiation) that is above 10⁷ (10 million) kelvins, and thermal emission from thick gases (called blackbody radiation) that are above 10⁷ Kelvin.^[21] Since X-rays are absorbed by the Earth's atmosphere, all X-ray observations must be done from high-altitude balloons, rockets, or spacecraft. Notable X-ray sources include X-ray binaries, pulsars, supernova remnants, elliptical galaxies, clusters of galaxies, and active galactic nuclei.^[21]

Gamma-ray astronomy

Main article: Gamma ray astronomy

Gamma ray astronomy is the study of astronomical objects at the shortest wavelengths of the electromagnetic spectrum. Gamma rays may be observed directly by satellites such as the Compton Gamma Ray Observatory or by specialized telescopes called atmospheric Cherenkov telescopes.^[21] The Cherenkov telescopes do not actually detect the gamma rays directly but instead detect the flashes of visible light produced when gamma rays are absorbed by the Earth's atmosphere.^[25]

Most gamma-ray emitting sources are actually gamma-ray bursts, objects which only produce gamma radiation for a few milliseconds to thousands of seconds before fading away. Only 10% of gamma-ray sources are non-transient sources. These steady gamma-ray emitters include pulsars, neutron stars, and black hole candidates such as active galactic nuclei.^[21]

Fields of observational astronomy not based on the electromagnetic spectrum

Other than electromagnetic radiation, few things may be observed from the Earth that originate from great distances.

In neutrino astronomy, astronomers use special underground facilities such as SAGE, GALLEX, and Kamioka II/III for detecting neutrinos. These neutrinos originate primarily from the Sun but also from supernovae.^[21]

Cosmic rays consisting of very high energy particles can be observed hitting the Earth's atmosphere.^{[citation needed]} Additionally, some future neutrino detectors will also be sensitive to the neutrinos produced when cosmic rays hit the Earth's atmosphere.^[21]

Gravitational wave astronomy is an emerging new window of astronomy, which aims to use gravitational wave detectors to collect observational data about compact objects. A few observatories have been constructed, such as the Laser Interferometer Gravitational Observatory LIGO, but gravitational waves are extremely difficult to detect.^[26]

Planetary astronomy has benefited from direct observation in the form of spacecraft and sample return missions. These include fly-by missions with remote sensors; landing vehicles that can perform experiments on the surface materials; impactors that allow remote sensing of buried material, and sample return missions that allow direct, laboratory examination.

Astrometry and celestial mechanics

Main articles: Astrometry and Celestial mechanics

One of the oldest fields in astronomy, and in all of science, is the measurement of the positions of celestial objects. Historically, accurate knowledge of the positions of the Sun, Moon, planets and stars has been essential in celestial navigation.

Careful measurement of the positions of the planets has led to a solid understanding of gravitational perturbations, and an ability to determine past and future positions of the planets with great accuracy, a field known as celestial mechanics. More recently the tracking of near-Earth objects will allow for predictions of close encounters, and potential collisions, with the Earth.^[27]

The measurement of stellar parallax of nearby stars provides a fundamental baseline in the cosmic distance ladder that is used to measure the scale of the universe. Parallax measurements of nearby stars provide an absolute baseline for the properties of more distant stars, because their properties can be compared. Measurements of radial velocity and proper motion show the kinematics of these systems through the Milky Way galaxy. Astrometric results are also used to measure the distribution of dark matter in the galaxy.^[28]

During the 1990s, the astrometric technique of measuring the stellar wobble was used to detect large extrasolar planets orbiting nearby stars.^[29]

Theoretical astronomy

Nucleosynthesis

Stellar nucleosynthesis Big Bang nucleosynthesis Supernova nucleosynthesis Cosmic ray spallation

Related topics

Astrophysics Nuclear fusion R-process S-process Nuclear fission

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Theoretical astronomers use a wide variety of tools which include analytical models (for example, polytropes to approximate the behaviors of a star) and computational numerical simulations. Each has some advantages. Analytical models of a process are generally better for giving insight into the heart of what is going on. Numerical models can reveal the existence of phenomena and effects that would otherwise not be seen.^[30][31]

Theorists in astronomy endeavor to create theoretical models and figure out the observational consequences of those models. This helps observers look for data that can refute a model or help in choosing between several alternate or conflicting models.

Theorists also try to generate or modify models to take into account new data. In the case of an inconsistency, the general tendency is to try to make minimal modifications to the model to fit the data. In some cases, a large amount of inconsistent data over time may lead to total abandonment of a model.

Topics studied by theoretical astronomers include: stellar dynamics and evolution; galaxy formation; large-scale structure of matter in the Universe; origin of cosmic rays; general relativity and physical cosmology, including string cosmology and astroparticle physics. Astrophysical relativity serves as a tool to gauge the properties of large scale structures for which gravitation plays a significant role in physical phenomena investigated and as the basis for black hole (astro)physics and the study of gravitational waves.

Some widely accepted and studied theories and models in astronomy, now included in the Lambda-CDM model are the Big Bang, Cosmic inflation, dark matter, and fundamental theories of physics.

A few examples of this process:

Physical process	Experimental tool	Theoretical model	Explains/predicts
Gravitation	Radio telescopes	Self-gravitating system	Emergence of a star system
Nuclear fusion	Spectroscopy	Stellar evolution	How the stars shine and how metals formed
The Big Bang	Hubble Space Telescope, COBE	Expanding universe	Age of the Universe
Quantum fluctuations		Cosmic inflation	Flatness problem
Gravitational collapse	X-ray astronomy	General relativity	Black holes at the center of Andromeda galaxy
CNO cycle in stars

Dark matter and dark energy are the current leading topics in astronomy, as their discovery and controversy originated during the study of the galaxies.

Specific subfields of astronomy

Solar astronomy

Main article: Sun

At a distance of about eight light-minutes, the most frequently studied star is the Sun, a typical main-sequence dwarf star of stellar class G2 V, and about 4.6 Gyr in age. The Sun is not considered a variable star, but it does undergo periodic changes in activity known as the sunspot cycle. This is an 11-year fluctuation in sunspot numbers. Sunspots are regions of lower-than- average temperatures that are associated with intense magnetic activity.^[32]

An ultraviolet image of the Sun's active photosphere as viewed by the TRACE space telescope. NASA photo.

The Sun has steadily increased in luminosity over the course of its life, increasing by 40% since it first became a main-sequence star. The Sun has also undergone periodic changes in luminosity that can have a significant impact on the Earth.^[33] The Maunder minimum, for example, is believed to have caused the Little Ice Age phenomenon during the Middle Ages.^[34]

The visible outer surface of the Sun is called the photosphere. Above this layer is a thin region known as the chromosphere. This is surrounded by a transition region of rapidly increasing temperatures, then by the super-heated corona.

At the center of the Sun is the core region, a volume of sufficient temperature and pressure for nuclear fusion to occur. Above the core is the radiation zone, where the plasma conveys the energy flux by means of radiation. The outer layers form a convection zone where the gas material transports energy primarily through physical displacement of the gas. It is believed that this convection zone creates the magnetic activity that generates sun spots.^[32]

A solar wind of plasma particles constantly streams outward from the Sun until it reaches the heliopause. This solar wind interacts with the magnetosphere of the Earth to create the Van Allen radiation belts, as well as the aurora where the lines of the Earth's magnetic field descend into the atmosphere.^[35]

Planetary science

Main articles: Planetary science and Planetary geology

This astronomical field examines the assemblage of planets, moons, dwarf planets, comets, asteroids, and other bodies orbiting the Sun, as well as extrasolar planets. The solar system has been relatively well-studied, initially through telescopes and then later by spacecraft. This has provided a good overall understanding of the formation and evolution of this planetary system, although many new discoveries are still being made.^[36]

The black spot at the top is a dust devil climbing a crater wall on Mars. This moving, swirling column of Martian atmosphere (comparable to a terrestrial tornado) created the long, dark streak. NASA image.

The solar system is subdivided into the inner planets, the asteroid belt, and the outer planets. The inner terrestrial planets consist of Mercury, Venus, Earth, and Mars. The outer gas giant planets are Jupiter, Saturn, Uranus, and Neptune.^[37] Beyond Neptune lies the Kuiper Belt, and finally the Oort Cloud, which may extend as far as a light-year.

The planets were formed by a protoplanetary disk that surrounded the early Sun. Through a process that included gravitational attraction, collision, and accretion, the disk formed clumps of matter that, with time, became protoplanets. The radiation pressure of the solar wind then expelled most of the unaccreted matter, and only those planets with sufficient mass retained their gaseous atmosphere. The planets continued to sweep up, or eject, the remaining matter during a period of intense bombardment, evidenced by the many impact craters on the Moon. During this period, some of the protoplanets may have collided, the leading hypothesis for how the Moon was formed.^[38]

Once a planet reaches sufficient mass, the materials with different densities segregate within, during planetary differentiation. This process can form a stony or metallic core, surrounded by a mantle and an outer surface. The core may include solid and liquid regions, and some planetary cores generate their own magnetic field, which can protect their atmospheres from solar wind stripping.^[39]

A planet or moon's interior heat is produced from the collisions that created the body, radioactive materials (e.g. uranium, thorium, and ²⁶Al), or tidal heating. Some planets and moons accumulate enough heat to drive geologic processes such as volcanism and tectonics. Those that accumulate or retain an atmosphere can also undergo surface erosion from wind or water. Smaller bodies, without tidal heating, cool more quickly; and their geological activity ceases with the exception of impact cratering.^[40]

Stellar astronomy

The Ant planetary nebula. Ejecting gas from the dying central star shows symmetrical patterns unlike the chaotic patterns of ordinary explosions.

Main article: Star

The study of stars and stellar evolution is fundamental to our understanding of the universe. The astrophysics of stars has been determined through observation and theoretical understanding; and from computer simulations of the interior.

Star formation occurs in dense regions of dust and gas, known as giant molecular clouds. When destabilized, cloud fragments can collapse under the influence of gravity, to form a protostar. A sufficiently dense, and hot, core region will trigger nuclear fusion, thus creating a main-sequence star.^[41]

Almost all elements heavier than hydrogen and helium were created inside the cores of stars.

The characteristics of the resulting star depend primarily upon its starting mass. The more massive the star, the greater its luminosity, and the more rapidly it expends the hydrogen fuel in its core. Over time, this hydrogen fuel is completely converted into helium, and the star begins to evolve. The fusion of helium requires a higher core temperature, so that the star both expands in size, and increases in core density. The resulting red giant enjoys a brief life span, before the helium fuel is in turn consumed. Very massive stars can also undergo a series of decreasing evolutionary phases, as they fuse increasingly heavier elements.

The final fate of the star depends on its mass, with stars of mass greater than about eight times the Sun becoming core collapse supernovae; while smaller stars form planetary nebulae, and evolve into white dwarfs. The remnant of a supernova is a dense neutron star, or, if the stellar mass was at least three times that of the Sun, a black hole.^[42] Close binary stars can follow more complex evolutionary paths, such as mass transfer onto a white dwarf companion that can potentially cause a supernova. Planetary nebulae and supernovae are necessary for the distribution of metals to the interstellar medium; without them, all new stars (and their planetary systems) would be formed from hydrogen and helium alone.

Galactic astronomy

Main article: Galactic astronomy

Observed structure of the Milky Way's spiral arms

Our solar system orbits within the Milky Way, a barred spiral galaxy that is a prominent member of the Local Group of galaxies. It is a rotating mass of gas, dust, stars and other objects, held together by mutual gravitational attraction. As the Earth is located within the dusty outer arms, there are large portions of the Milky Way that are obscured from view.

In the center of the Milky Way is the core, a bar-shaped bulge with what is believed to be a supermassive black hole at the center. This is surrounded by four primary arms that spiral from the core. This is a region of active star formation that contains many younger, population I stars. The disk is surrounded by a spheroid halo of older, population II stars, as well as relatively dense concentrations of stars known as globular clusters.^[43][44]

Between the stars lies the interstellar medium, a region of sparse matter. In the densest regions, molecular clouds of molecular hydrogen and other elements create star-forming regions. These begin as irregular dark nebulae, which concentrate and collapse (in volumes determined by the Jeans length) to form compact protostars.^[45]

As the more massive stars appear, they transform the cloud into an H II region of glowing gas and plasma. The stellar wind and supernova explosions from these stars eventually serve to disperse the cloud, often leaving behind one or more young open clusters of stars. These clusters gradually disperse, and the stars join the population of the Milky Way.

Kinematic studies of matter in the Milky Way and other galaxies have demonstrated that there is more mass than can be accounted for by visible matter. A dark matter halo appears to dominate the mass, although the nature of this dark matter remains undetermined.^[46]

Extragalactic astronomy

Main article: Extragalactic astronomy

The study of objects outside of our galaxy is a branch of astronomy concerned with the formation and evolution of Galaxies; their morphology and classification; and the examination of active galaxies, and the groups and clusters of galaxies. The latter is important for the understanding of the large-scale structure of the cosmos.

This image shows several blue, loop-shaped objects that are multiple images of the same galaxy, duplicated by the gravitational lens effect of the cluster of yellow galaxies near the middle of the photograph. The lens is produced by the cluster's gravitational field that bends light to magnify and distort the image of a more distant object.

Most galaxies are organized into distinct shapes that allow for classification schemes. They are commonly divided into spiral, elliptical and Irregular galaxies.^[47]

As the name suggests, an elliptical galaxy has the cross-sectional shape of an ellipse. The stars move along random orbits with no preferred direction. These galaxies contain little or no interstellar dust; few star-forming regions; and generally older stars. Elliptical galaxies are more commonly found at the core of galactic clusters, and may be formed through mergers of large galaxies.

A spiral galaxy is organized into a flat, rotating disk, usually with a prominent bulge or bar at the center, and trailing bright arms that spiral outward. The arms are dusty regions of star formation where massive young stars produce a blue tint. Spiral galaxies are typically surrounded by a halo of older stars. Both the Milky Way and the Andromeda Galaxy are spiral galaxies.

Irregular galaxies are chaotic in appearance, and are neither spiral nor elliptical. About a quarter of all galaxies are irregular, and the peculiar shapes of such galaxies may be the result of gravitational interaction.

An active galaxy is a formation that is emitting a significant amount of its energy from a source other than stars, dust and gas; and is powered by a compact region at the core, usually thought to be a super-massive black hole that is emitting radiation from in-falling material.

A radio galaxy is an active galaxy that is very luminous in the radio portion of the spectrum, and is emitting immense plumes or lobes of gas. Active galaxies that emit high-energy radiation include Seyfert galaxies, Quasars, and Blazars. Quasars are believed to be the most consistently luminous objects in the known universe.^[48]

The large-scale structure of the cosmos is represented by groups and clusters of galaxies. This structure is organized in a hierarchy of groupings, with the largest being the superclusters. The collective matter is formed into filaments and walls, leaving large voids in between.^[49]

Cosmology

Main article: Physical cosmology

Cosmology (from the Greek κοσμος "world, universe" and λογος "word, study") could be considered the study of the universe as a whole.

Observations of the large-scale structure of the universe, a branch known as physical cosmology, have provided a deep understanding of the formation and evolution of the cosmos. Fundamental to modern cosmology is the well-accepted theory of the big bang, wherein our universe began at a single point in time, and thereafter expanded over the course of 13.7 Gyr to its present condition. The concept of the big bang can be traced back to the discovery of the microwave background radiation in 1965.

In the course of this expansion, the universe underwent several evolutionary stages. In the very early moments, it is theorized that the universe experienced a very rapid cosmic inflation, which homogenized the starting conditions. Thereafter, nucleosynthesis produced the elemental abundance of the early universe. (See also nucleocosmochronology.)

When the first atoms formed, space became transparent to radiation, releasing the energy viewed today as the microwave background radiation. The expanding universe then underwent a Dark Age due to the lack of stellar energy sources.^[50]

A hierarchical structure of matter began to form from minute variations in the mass density. Matter accumulated in the densest regions, forming clouds of gas and the earliest stars. These massive stars triggered the reionization process and are believed to have created many of the heavy elements in the early universe which tend to decay back to the lighter elements extending the cycle.

Gravitational aggregations clustered into filaments, leaving voids in the gaps. Gradually, organizations of gas and dust merged to form the first primitive galaxies. Over time, these pulled in more matter, and were often organized into groups and clusters of galaxies, then into larger-scale superclusters.^[51]

Fundamental to the structure of the universe is the existence of dark matter and dark energy. These are now thought to be the dominant components, forming 96% of the density of the universe. For this reason, much effort is expended in trying to understand the physics of these components.^[52]

Interdisciplinary studies

Astronomy and astrophysics have developed significant interdisciplinary links with other major scientific fields. Archaeoastronomy is the study of ancient or traditional astronomies in their cultural context, utilizing archaeological and anthropological evidence. Astrobiology is the study of the advent and evolution of biological systems in the universe, with particular emphasis on the possibility of non-terrestrial life.

The study of chemicals found in space, including their formation, interaction and destruction, is called astrochemistry. These substances are usually found in molecular clouds, although they may also appear in low temperature stars, brown dwarfs and planets. Cosmochemistry is the study of the chemicals found within the Solar System, including the origins of the elements and variations in the isotope ratios. Both of these fields represent an overlap of the disciplines of astronomy and chemistry.

Amateur astronomy

Main article: Amateur astronomy

Amateur astronomers can build their own equipment, and can hold star parties and gatherings, such as Stellafane.

Astronomy is one of the sciences to which amateurs can contribute the most^[53].

Collectively, amateur astronomers observe a variety of celestial objects and phenomena sometimes with equipment that they build themselves. Common targets of amateur astronomers include the Moon, planets, stars, comets, meteor showers, and a variety of deep-sky objects such as star clusters, galaxies, and nebulae. One branch of amateur astronomy, amateur astrophotography, involves the taking of photos of the night sky. Many amateurs like to specialize in the observation of particular objects, types of objects, or types of events which interest them.^[54][55]

Most amateurs work at visible wavelengths, but a small minority experiment with wavelengths outside the visible spectrum. This includes the use of infrared filters on conventional telescopes, and also the use of radio telescopes. The pioneer of amateur radio astronomy was Karl Jansky who started observing the sky at radio wavelengths in the 1930s. A number of amateur astronomers use either homemade telescopes or use radio telescopes which were originally built for astronomy research but which are now available to amateurs (e.g. the One-Mile Telescope).^[56][57]

Amateur astronomers continue to make scientific contributions to the field of astronomy. Indeed, it is one of the few scientific disciplines where amateurs can still make significant contributions. Amateurs can make occultation measurements that are used to refine the orbits of minor planets. They can also discover comets, and perform regular observations of variable stars. Improvements in digital technology have allowed amateurs to make impressive advances in the field of astrophotography.^[58][59][60]

Major questions in astronomy

See also: Unsolved problems in physics

Although the scientific discipline of astronomy has made tremendous strides in understanding the nature of the universe and its contents, there remain some important unanswered questions. Answers to these may require the construction of new ground- and space-based instruments, and possibly new developments in theoretical and experimental physics.

• What is the origin of the stellar mass spectrum? That is, why do astronomers observe the same distribution of stellar masses—the initial mass function—apparently regardless of the initial conditions?^[61] A deeper understanding of the formation of stars and planets is needed.
• Is there other life in the Universe? Especially, is there other intelligent life? If so, what is the explanation for the Fermi paradox? The existence of life elsewhere has important scientific and philosophical implications.^[62][63]
• What is the nature of dark matter and dark energy? These dominate the evolution and fate of the cosmos, yet we are still uncertain about their true natures.^[64]
• Why are the physical constants so finely tuned that they permit the existence of life? Could they be the result of cosmological natural selection? What caused the cosmic inflation that produced our homogeneous universe?
• What will be the ultimate fate of the universe?^[65]

The International Year of Astronomy 2009

Main article: International Year of Astronomy

During the 62nd General Assembly of the UN, 2009 was declared to be the International Year of Astronomy (IYA2009), with the resolution being made official on 20 December 2008. A global scheme laid out by the International Astronomical Union (IAU), it has also been endorsed by UNESCO — the UN body responsible for Educational, Scientific and Cultural matters. IYA2009 is intended to be a global celebration of astronomy and its contributions to society and culture, stimulating worldwide interest not only in astronomy but science in general, with a particular slant towards young people.

See also

Astronomy portal

Space portal

Wikipedia:Books has a book on: Astronomy

Astrology and astronomy Astronomer Astrophysics Data System Category:Ancient astronomy Category:Astronomical observatories Category:Astronomy organizations Category:Astronomy timelines Constellation

Cosmogony Extragalactic Distance Scale International Year of Astronomy List of astronomy acronyms List of basic astronomy topics Solar System Space exploration Space science Telescope

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External links

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Learning resources from Wikiversity

• http://www.nasa.gov/
• International Year of Astronomy 2009 IYA2009 Main website
• Cosmic Journey: A History of Scientific Cosmology from the American Institute of Physics
• Astronomy Picture of the Day
• Southern Hemisphere Astronomy
• Sky & Telescope publishers
• Astronomy Magazine
• Universe Today for astronomy and space-related news
• Celestia Motherlode Educational site for Astronomical journeys through space
• Hubblesite.org - home of NASA's Hubble Space Telescope
• Astronomy - A History - G. Forbes - 1909 (eLibrary Project - eLib Text)
• (historical)
• Prof. Sir Harry Kroto, NL, Astrophysical Chemistry Lecture Series. 8 Freeview Lectures provided by the Vega Science Trust.
• Core books and core journals in Astronomy, from the Smithsonian/NASA Astrophysics Data System
• Astronomy Magazine, Astronomical Reviews
• Galaxy Zoo

v • d • e General subfields within astronomy and astrophysics Cosmology · Extragalactic astronomy · Galactic astronomy · Stellar astronomy · Planetary science · Exoplanetology · Astrogeology · Astrometry · Astrochemistry · Astrobiology  · Astrodynamics

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Dansk (Danish)
n. - astronomi

Nederlands (Dutch)
astronomie, sterrenkunde

Français (French)
n. - astronomie

Deutsch (German)
n. - Astronomie, Sternkunde

Ελληνική (Greek)
n. - αστρονομία

Italiano (Italian)
astronomia

Português (Portuguese)
n. - astronomia (f)

Русский (Russian)
астрономия

Español (Spanish)
n. - astronomía

Svenska (Swedish)
n. - astronomi

中文（简体）(Chinese (Simplified))
天文学

中文（繁體）(Chinese (Traditional))
n. - 天文學

한국어 (Korean)
n. - 천문학

日本語 (Japanese)
n. - 天文学

العربيه (Arabic)
‏(الاسم) علم الفضا, علم الفلك, رساله في علم الفلك‏

עברית (Hebrew)
n. - ‮אסטרונומיה‬

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