ARISTARCHUS OF SAMOS

'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 pythagoréan 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.

Contents

Heliocentrism

Distance to the Sun

The Great Year and the First High Precision Estimate of the Length of the Month

Precession

External links

References

Further reading

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)
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''):
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

Main articles: Aristarchus On the Sizes and Distances

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

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★ Biography: JRASC, '75' (1981) 29

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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