Aristarchus (Greek: Ἀρίσταρχος; 310 BC - ca.
230 BC) was a Greek astronomer and mathematician, born on the island of Samos, in ancient Greece. He was the first person to present an
explicit argument for a heliocentric model of the solar
system, placing the Sun, not the Earth, at
the center of the known universe (hence he is sometimes known as the "Greek Copernicus"). He was influenced by his teacher, the Pythagorean Philolaus of Kroton, but in contrast to Philolaus he had
both identified the central fire with the Sun, as well as putting other planets in correct order from the Sun. His astronomical
ideas were rejected in favor of the geocentric theories of Aristotle and Ptolemy until they were successfully revived and
extensively developed by Copernicus nearly 2000 years later.
The Aristarchus crater on the Moon was named in
his honour.
Heliocentrism
The only work usually attributed to Aristarchus which has survived to the present time, On the Sizes and Distances of the Sun and Moon, is based on a geocentric
world view. It is peculiar and possibly informative that this work reckons the sun's diameter as 2 degrees, rather the correct value,
1/2 degree. The latter diameter is known from Archimedes to have been Aristarchus's actual value.
Though the original text has been lost, a reference in Archimedes' book
The Sand Reckoner describes another work by Aristarchus in which he advanced an
alternative hypothesis of the heliocentric model. Archimedes wrote: (translated into
English)
You King Gelon are aware the 'universe' is the name given by most astronomers to the sphere the center of which is the center
of the Earth, while its radius is equal to the straight line between the center of the Sun and the center of the Earth. This is
the common account as you have heard from astronomers. But Aristarchus has brought out a book consisting of certain hypotheses,
wherein it appears, as a consequence of the assumptions made, that the universe is many times greater than the 'universe' just
mentioned. His hypotheses are that the fixed stars and the Sun remain unmoved, that the Earth revolves about the Sun on the
circumference of a circle, the Sun lying in the middle of the orbit, and that the sphere of fixed stars, situated about the same center as the Sun, is so great that the circle in which he supposes the Earth to
revolve bears such a proportion to the distance of the fixed stars as the center of the sphere bears to its surface.
Aristarchus thus believed the stars to be very far away, and saw this as the reason why there was no visible parallax, that is, an observed movement of the stars relative to each other as the Earth moved around the Sun.
The stars are in fact much farther away than the distance that was assumed in ancient times, which is why stellar parallax is
only detectable with telescopes. But the awkwardly complex geocentric model, designed to evade recognition of plainly visible planetary parallax, was
assumed to be a more comfortable explanation for the unobservability of the parallel phenomenon, stellar parallax. The
rejection of the heliocentric view was apparently quite strong, as the following passage from Plutarch suggests (On the Apparent Face in the Orb of the Moon):
Cleanthes [a contemporary of Aristarchus and head of the Stoics] thought it was the duty of
the Greeks to indict Aristarchus of Samos on the charge of impiety for putting in motion the Hearth of the universe [i.e. the
earth], . . . supposing the heaven to remain at rest and the earth to revolve in an oblique circle, while it rotates, at the same
time, about its own axis.[1]
His hypothesis no longer fit the Greek conceptual
system, which had left the speculative period and entered a more authoritative phase. He was struck by a problem that
would trickle through the entire history of philosophy – the tension between
conservatism and revolution, between what is
regarded as certain knowledge and what is regarded as unknowable. The most earnest discussions along these lines within modern
epistemology have been entertained by Thomas
Kuhn, Karl Popper, Imre Lakatos,
Paul Feyerabend and lately also by the Swedish philosopher Sören Halldén (2005). The
pioneering giant Aristarchus's ideas fell into oblivion because they led away from the main-stream that had already been laid
down by the less perceptive school forming giants of philosophy, Plato and Aristotle. The only other astronomer from antiquity who is known by name and who is known to have supported
Aristarchus' heliocentric model was Seleucus of Seleucia, a Mesopotamian astronomer who lived a century after Aristarchus.
Distance to the Sun
Aristarchus's 3rd century BC calculations on the relative sizes of the Earth, Sun and Moon, from a 10th century CE Greek
copy
-
Aristarchus claimed to have observed that at half moon (first or last quarter moon), the
angle between sun and moon was 87°. Using correct geometry, but insufficiently accurate
observational data, Aristarchus concluded that the Sun was 19 times farther away than the Moon. (The true value of this angle is
close to 89° 50', and the Sun is actually about 390 times farther away.) The implicit false solar parallax of slightly under 3'
was used by astronomers up to and including Brahe, ca. 1600 A. D. Aristarchus pointed out that the Moon and Sun have nearly equal
apparent angular sizes and therefore their diameters must be in proportion to their distances from
Earth. He thus concluded that the diameter of the Sun was 20 times larger than the diameter of the Moon; which, although wrong,
follows logically from his data. It also leads to the conclusion that the Sun's diameter is almost seven times greater than the
Earth's; the volume of Aristarchus's Sun would be almost 300 times greater than the Earth. Perhaps this difference in sizes
inspired the heliocentric model.
The Great Year and the First High Precision Estimate of the Length of the Month
Admired by Archimedes and by modern scientists for having the vision to be the first to propose a huge universe, Aristarchus
also proposed the largest ancient Greek time period, his well-known "Great Year" of 4868 solar years, equalling exactly 270
saroi, each of 18 Callippic years plus 10 and 2/3 degrees. (Syntaxis book 4 chapter 2.) Its empirical foundation was the
famous, usefully stable 4267 month eclipse cycle, cited by Ptolemy as source of the extremely accurate Babylonian month, which
was good to a fraction of a second (1 part in several million), and is found on cuneiform tablets from shortly before 200 B. C.
(Due to near integral returns in lunar and solar anomaly, eclipses 4267 months apart exceptionally never deviated by more than an
hour from a mean of 126007 days plus 1 hour, the value given by Ptolemy at op cit. Thus, estimation of the length of the
month was ensured to have relative accuracy of 1 part in millions.) Embedded in the Great Year was a length of the month agreeing
with the Babylonian value to 1 part in tens of millions, decades before Babylon is known to have used it. Aristarchus's work
represents an advance of science in several respects. Previous estimates of the length of the month were in error by 114 seconds
(Meton, 432 B. C.) and 22 seconds (Callippus, 330 B.
C.). The attribution of a reliable mean motion to so complex a motion as the moon's was a remarkable conceptual leap.
Precession
The Vatican has preserved two ancient manuscripts of estimates of the length of the year. The
only ancient scientist listed for two different values is Aristarchus. It is now widely suspected that these are among the
earliest surviving examples of continued fraction expressions. The most obvious
interpretations are precisely computable from the manuscript numbers. The results are Aristarchus years of 365 days plus 1/152,
and 365 days minus 15/4868, representing the sidereal year and the civil, supposedly tropical year. Both denominators are
relatable to Aristarchus, whose summer solstice was 152 years after Meton's and whose
Great Year was 4868 years. The difference between the sidereal and tropical year is identical to precession. The former value is accurate within a few seconds. The latter is erroneous by several
minutes. Both are close to the values later used by Hipparchus and Ptolemy, and the precession indicated is almost precisely 1 degree per century, a much-too-low value.
Unfortunately, 1 degree per century precession was used by all later astronomers until the Arabs. The correct value in
Aristarchus's time was about 1.38 degrees per century.
External links
References
- Heath, Sir Thomas. Aristarchus of Samos - The Ancient Copernicus, A history of Greek
astronomy to Aristarchus together with Aristarchus' treatise on the sizes and distances of the sun and moon, a new Greek text
with translation and notes. (ISBN 0-486-43886-4)
Further reading
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