ephemeris

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An ephemeris (plural: ephemerides; from the Greek word ἐφημερίς ephēmeris "diary", "journal") is a table of values that gives the positions of astronomical objects in the sky at a given time or times. Different kinds of ephemerides are used for astronomy and astrology. Even though this was also one of the first applications of mechanical computers, an ephemeris will still often be a simple printed table.

The position is given to astronomers in a spherical polar coordinate system of right ascension and declination or to astrologers in longitude along the zodiacal ecliptic, and sometimes declination. Astrological positions may be given for either noon or midnight.

An astronomical ephemeris may also provide data on astronomical phenomena of interest to astrologers and astronomers such as eclipses, Apparent retrograde motion/planetary stations, planetary ingresses, sidereal time, positions for the mean and true nodes of the moon, the phases of the Moon, and sometimes even the position(s) of Chiron, and other minor celestial bodies.

Astrologers also use other ephemerides that include Lilith, a term they use variously for the apogee of the Moon or the second focus of the Moon's orbit.^[1] Some ephemerides also contain a monthly aspectarian, while others often include the declination of the planets as well as their longitudes, right ascensions, or Cartesian coordinates.^{[citation needed]}

History

A Latin translation of al-Khwārizmī's Zīj, page from Corpus Christi College MS 283

Alfonsine tables

Page from Almanach Perpetuum

• 2nd millennium BC — Panchanga tables based on Jyotisha in the Vedic period of Indian astronomy.
• 1st millennium BC — Ephemerides in Babylonian astronomy.
• 2nd century AD — the Almagest and the Handy Tables of Ptolemy
• 8th century AD — the Zij of Ibrāhīm al-Fazārī
• 9th century AD — the Zij of Muḥammad ibn Mūsā al-Khwārizmī
• 12th century AD — the Tables of Toledo, based largely on Arabic Zij sources of Islamic astronomy, were edited by Gerard of Cremona to form the standard European ephemeris until the Alfonsine tables.
• 13th century — the Zij-i Ilkhani, or Ilkhanic Tables, were compiled at the Maragheh observatory in Persia.
• 13th century — the Alfonsine tables were compiled in Spain to correct anomalies in the Tables of Toledo, remaining the standard European ephemeris until the Prutenic Tables almost 300 years later.
• 1408 — Chinese Ephemeris Table (copy in Pepysian Library, Cambridge, UK (refer book '1434'); Chinese tables believed known to Regiomontanus).
• 1496 — the Almanach Perpetuum of Abraão ben Samuel Zacuto (one of the first books published with a movable type and printing press in Portugal)
• 1504 — While shipwrecked on the island of Jamaica, Christopher Columbus successfully predicted a lunar eclipse for the natives, using the Ephemeris of the German astronomer Regiomontanus.
• 1551 — the Prutenic Tables of Erasmus Reinhold were published, based on Copernicus's theories.
• 1554 — Johannes Stadius published a well-known work known as Ephemerides novae at auctae that attempted to give accurate planetary positions. The effort was not entirely successful, and there were, for example, periodic errors in Stadius’ Mercury positions of up to ten degrees.
• 1627 — the Rudolphine Tables of Johannes Kepler became the new standard.

Scientific ephemeris

For scientific uses, a modern planetary ephemeris comprises software that generates positions of the planets and often of their satellites, or of asteroids or comets at virtually any time desired by the user. Often there is an option to find the velocities of the bodies of interest, as well.

Typically, such ephemerides cover several centuries, past and future; the future ones can be covered because celestial mechanics is an accurate theory. Nevertheless, there are secular phenomena which cannot adequately be considered by ephemerides. The biggest uncertainties on planetary positions are due to the perturbations of numerous asteroids, most of whose masses and orbits are poorly known, rendering their effect uncertain. Reflecting the continuing influx of new data and observations, the JPL has to revise its published ephemerides at intervals of 20 years.^[2]

Solar system ephemerides are essential for the navigation of spacecraft and for all kinds of space observations of the planets, their natural satellites, stars and galaxies.

Scientific ephemerides for sky observers mostly contain the position of the mentioned celestial body in right ascension and declination, because these coordinates are the most often used on star maps and telescopes. The equinox of the coordinate system must be given. It is in nearly all cases either the actual equinox (the equinox valid for that moment, often referred to as "of date" or "current"), or that of one of the "standard" equinoxes, typically J2000.0, B1950.0, or J1900. Star maps are almost always in one of the standard equinoxes.

Scientific ephemerides often contain further useful data about the moon, planet, asteroid, or comet beyond the pure coordinates in the sky, such as elongation to the sun, brightness, distance, velocity, apparent diameter in the sky, phase angle, times of rise, transit, and set, etc. Ephemerides of the planet Saturn also sometimes contain the apparent inclination of its ring.

An ephemeris is usually only correct for a particular location on the Earth. In many cases the differences are too small to matter, but for nearby asteroids or the Moon they can be quite important.

Global Positioning System (GPS) navigation satellites transmit electronic ephemeris data consisting of health and exact location data that GPS receivers then use (together with the signal's elapsed travel time to the receiver) to calculate their own location on Earth using trilateration.

Other modern ephemerides recently created are the EPM due to the Russian Institute for Applied Astronomy of the Russian Academy of Sciences, and the INPOP ones by the French IMCCE.

Astrological ephemeris

The majority of astrologers study tropical astrology, involving planetary positions referenced to the vernal (spring) equinox position along the ecliptic (the equinox being the center of Earth's rotational plane and Earth's orbital plane around the Sun). They use exactly the same referential frame of the astronomers, except for astrologers who study sidereal astrology (Indian Astrology) and use a different ephemerids, based on the constellations.

Though astrology is and always has been geocentric, heliocentric astrology is an emerging field; for this purpose a standard ephemeris cannot be utilized, and because of this specialized heliocentric ephemerides must be calculated and used instead of the default geocentric ephemerides that are used in standard Western astrology to construct the astrological chart/natal chart.

See also

• The Astronomical Almanac (new name)
• American Ephemeris and Nautical Almanac (old name)
• Ephemeris time
• Epoch (astronomy)
• Epoch (reference date)
• January 0 or March 0
• Jet Propulsion Laboratory Development Ephemeris
• Fundamental ephemeris
• Almanac

Notes

References

• Duffett-Smith, Peter (1990). Astronomy With Your Personal Computer. Cambridge University Press. ISBN 0-521-38995-X. 
• MacCraig, Hugh (1949). The 200 Year Ephemeris. Macoy Publishing Company. 
• Meeus, Jean (1991). Astronomical Algorithms. Willmann-Bell. ISBN 0-943396-35-2. 
• Michelsen, Neil F. (1990). Tables of Planetary Phenomena. ACS Publications, Inc.. ISBN 0-935127-08-9. 
• Michelsen, Neil F. (1982). The American Ephemeris for the 21st Century - 2001 to 2100 at Midnight. Astro Computing Services. ISBN 0-917086-50-3. 
• Montenbruck, Oliver (1989). Practical Ephemeris Calculations. Springer-Verlag. ISBN 0-387-50704-3. 
• Seidelmann, Kenneth (2006). Explanatory supplement to the astronomical almanac. University Science Books. ISBN 1-891389-45-9. 

External links

Wikimedia Commons has media related to: Ephemeris

• The JPL HORIZONS online ephemeris
• Introduction to the JPL ephemerides
• The effect of asteroidal perturbations on the long term accuracy of ephemerides
• Kharin, A. and Kolesnik, Y.; On the Errors of the Ephemerides Derived from Optical Observations of Planets. (1990), IAU SYMP.141 P.189, 1989.
• Source code for computing ephemerides in C++ - by Steve Moshier
• Source code for easy and accurate ephemerides and charts in Java - by Tomás Alonso
• Interactive orrery and ephemeris provided by Fourmilab in Switzerland.
• Daily ephemeris, at the TAU AstroClub web site
• JavaScript Ephemeris
• The Photographer’s Ephemeris Adobe AIR application

Astrology links

• A Free 6000 Year Ephemeris Provided by Astro.com -- Based on Swiss Ephemeris, by Astrodienst Zürich, Switzerland (available in 8 languages)
• The Original 3,000 Year High-Precision Daily Astrological Online Ephemeris from Khaldea.com -- 600BC to 2400AD—Calculated for Midnight GMT; also with an Aspectarian included for years 1900 to 2005

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