# Aristarchus of Samos

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Aristarchus of Samos
Born c. 310 BC
Died c. 230 BC
Ethnicity Greek
Occupation Scholar, mathematician, astronomer

Aristarchus of Samos (; Greek: Ἀρίσταρχος Aristarkhos; c. 310 – c. 230 BC) was an ancient Greek astronomer and mathematician who presented the first known model that placed the Sun at the center of the known universe with the Earth revolving around it (see Solar system). He was influenced by Philolaus of Croton, but he identified the "central fire" with the Sun, and put the other planets in their correct order of distance around the Sun.[1] As Anaxagoras before him, he also suspected that the stars were just other bodies like the sun. His astronomical ideas were often rejected in favor of the geocentric theories of Aristotle and Ptolemy.

## Heliocentrism

Though the original text has been lost, a reference in Archimedes' book The Sand Reckoner (Archimedis Syracusani Arenarius & Dimensio Circuli) describes another work by Aristarchus in which he advanced the heliocentric model as an alternative hypothesis. Archimedes wrote:

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 Floor, and that the sphere of the 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.[2]

Aristarchus suspected the stars were other suns[3] that are very far away, and that in consequence there was no observable parallax, that is, a movement of the stars relative to each other as the Earth moves around the Sun. The stars are much farther away than was generally assumed in ancient times; and since stellar parallax is only detectable with telescopes, his speculation although accurate was unprovable at the time.

The geocentric model was consistent with planetary parallax and was assumed to be the reason why no stellar parallax was observed. As known, Ptolemy later preferred the geocentric model which was held for true during the Middle Age.

A demonstration of Aristarchus' heliocentric model was given by Seleucus of Seleucia, a Hellenistic astronomer who lived a century after Aristarchus.[4] The fact that Pliny the Elder[5] and Seneca[6] still referred to planets' retrograde motion as an apparent phenomenon, suggests that heliocentrism was an accepted theory still by their times.

It is a common idea that the heliocentric view was rejected by the contemporaries of Aristarchus. This is due to Gilles Ménage's translation of a passage from Plutarch's On the Apparent Face in the Orb of the Moon. Plutarch reported that Cleanthes (a contemporary of Aristarchus and head of the Stoics) as a worshipper of the Sun and opponent to the heliocentric model, was jokingly told by Aristarchus that he should be charged of impiety. Gilles Ménage, short after the Galileo and Giordano Bruno processes, amended an accusative with a nominative, and vice versa, so that the impiety accusation fell over the heliocentric sustainer. The resulting conception of an isolated and persecuted Aristarchus is still transmitted today.[7]

The heliocentric theory was successfully revived by Copernicus, after which Johannes Kepler described planetary motions with greater accuracy, with Kepler's laws, and Isaac Newton gave a theoretical explanation based on laws of gravitational attraction and dynamics.

## Distance to the Sun (lunar dichotomy)

Aristarchus's 3rd-century BC calculations on the relative sizes of (from left) the Sun, Earth and Moon, from a 10th-century AD Greek copy

The only surviving work usually attributed to Aristarchus, On the Sizes and Distances of the Sun and Moon, is based on a geocentric world view. It has historically been read as stating that the angle subtended by the Sun's diameter is 2 degrees, but Archimedes states in The Sand Reckoner that Aristarchus had a value of ½ degree, which is much closer to the actual average value of 32' or 0.53 degrees. The discrepancy may come from a misinterpretation of what unit of measure was meant by a certain Greek term in Aristarchus' text.[8]

Aristarchus claimed that at half moon (first or last quarter moon), the angle between Sun and Moon was 87°.[9] Possibly he proposed 87° as a lower bound since gauging the lunar terminator's deviation from linearity to 1° accuracy is beyond the unaided human ocular limit (that limit being about 3° accuracy). Aristarchus is known to have also studied light and vision.[10]

Using correct geometry, but the insufficiently accurate 87° datum, Aristarchus concluded that the Sun was between 18 and 20 times farther away than the Moon. (The true value of this angle is close to 89° 50', and the Sun's distance is actually about 400 times the Moon's.) The implicit false solar parallax of slightly under 3° was used by astronomers up to and including Tycho Brahe, ca. AD 1600. 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, so that the diameter of the Sun was between 18 and 20 times larger than the diameter of the Moon.[11]

## Notes

1. ^ Draper, John William, "History of the Conflict Between Religion and Science" in Joshi, S. T., 1874 (2007). The Agnostic Reader. Prometheus. pp. 172–173. ISBN 978-1-59102-533-7.
2. ^
3. ^ Louis Strous. "Who discovered that the Sun was a star?". solar-center.stanford.edu. Retrieved 2014-07-13.
4. ^ Plutarch, Platonicae quaestiones, VIII, i
5. ^ Naturalis historia, II, 70
6. ^ Naturales quaestiones, VII, xxv, 6-7
7. ^ Lucio Russo, Silvio M. Medaglia, Sulla presunta accusa di empietà ad Aristarco di Samo, in "Quaderni urbinati di cultura classica", n.s. 53 (82) (1996), pp. 113-121
8. ^ http://www.dioi.org/vols/we0.pdf
9. ^ Greek Mathematical Works, Loeb Classical Library, Harvard University, 1939–1941, edited by Ivor Thomas, volume 2 (1941), pages 6–7
10. ^ Heath, 1913, pp. 299–300; Thomas, 1942, pp. 2–3.
11. ^ Kragh, Helge (2007). Conceptions of cosmos: from myths to the accelerating universe: a history of cosmology. Oxford University Press. p. 26. ISBN 0-19-920916-2.