Aristarchus of Samos

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Aristarchus of Samos (Template:IPAc-en; Template:Langx, Template:Tlit; Template:Circa) was an ancient Greek astronomer and mathematician who presented the first known heliocentric model that placed the Sun at the center of the universe, with the Earth revolving around the Sun once a year and rotating about its axis once a day. He also supported the theory of Anaxagoras according to which the Sun was just another star.<ref>[1]</ref>

He likely moved to Alexandria, and he was a student of Strato of Lampsacus, who later became the third head of the Peripatetic school in Greece. According to Ptolemy, he observed the summer solstice of 280 BC.<ref>Template:Cite journal</ref> Along with his contributions to the heliocentric model, as reported by Vitruvius, he created two separate sundials: one that is a flat disc; and one hemispherical.<ref>{{#invoke:citation/CS1|citation |CitationClass=web }}</ref>

Aristarchus estimated the sizes of the Sun and Moon as compared to Earth's size. He also estimated the distances from the Earth to the Sun and Moon. He estimated that the Sun is seven times the size of Earth. Although this estimate is inaccurate by an order of magnitude, his insight that the Sun is larger than the Earth convinced him that the Sun must be the center of the universe.

Aristarchus was influenced by the concept presented by Philolaus of Croton (c. 470 – 385 BC) of a fire at the center of the universe (i.e. by contemporary understanding, at the center of the Earth). Aristarchus recast this "central fire" as the Sun, and he arranged the other planets in their correct order of distance around the Sun.<ref>Template:Cite book</ref>

Like Anaxagoras before him, Aristarchus suspected that the stars were just other bodies like the Sun, albeit farther away from Earth. His astronomical ideas were often rejected in favor of the geocentric theories of Aristotle and Ptolemy. Nicolaus Copernicus knew that Aristarchus had a 'moving Earth' theory, although it is unlikely that Copernicus was aware that it was a heliocentric theory.Template:Refn<ref name="Kish1978">For a (less recent) contrary view that Copernicus did know about Aristarchus's heliocentric theory see: Template:Cite book The Philolaus-Aristarchus passage is then given in untranslated Latin, without further comment. This is then followed by quoting in full Archimedes's passage about Aristarchus's heliocentric theory from 'The Sand Reckoner' (using its alternative title Arenarius)', seemingly without mentioning that The Sand Reckoner was not in print until a year after Copernicus's death (unless this is mentioned in a passage not shown by Google Books.).</ref>

Aristarchus is considered one of the greatest astronomers of antiquity along with Hipparchus.

HeliocentrismEdit

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The original text has been lost, but a reference in a book by Archimedes, entitled The Sand Reckoner (Archimedis Syracusani Arenarius & Dimensio Circuli), describes a work in which Aristarchus advanced the heliocentric model as an alternative hypothesis to geocentrism:

You are now aware ['you' being King Gelon] that the "universe" is the name given by most astronomers to the sphere the centre of which is the centre of the earth, while its radius is equal to the straight line between the centre of the sun and the centre of the earth. This is the common account (τὰ γραφόμενα) 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 the fixed stars, situated about the same centre 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 centre of the sphere bears to its surface.<ref>Heath, Thomas (1913), p. 302. The italics and parenthetical comments are as they appear in Thomas Little Heath's original. From Arenarius, 4–5. In the original: "κατέχεις δέ, διότι καλείται κόσμος ὑπὸ μὲν τῶν πλείστων ἀστρολόγων ἁ σφαῖρα, ἇς ἐστι κέντρον μὲν τὸ τᾶς γᾶς κέντρον, ἁ δὲ ἐκ τοῦ κέντρου ἴσα τᾷ εὐθείᾳ τᾷ μεταξὺ τοῦ κέντρου τοῦ ἁλίου καὶ τοῦ κέντρου τᾶς γᾶς. ταῦτα γάρ ἐντι τὰ γραφόμενα, ὡς παρὰ τῶν ἀστρολόγων διάκουσας. ̓Αρίσταρχος δὲ ό Σάμιος ὑποθεσίων τινων ἐξέδωκεν γραφάς, ἐν αἷς ἐκ τῶν ὑποκειμένων συμβαίνει τὸν κόσμον πολλαπλάσιον εἶμεν τοῦ νῦν εἰρημένου. ὑποτιθέται γὰρ τὰ μὲν ἀπλανέα τῶν ἄστρων καὶ τὸν ἅλιον μένειν ἀκίνητον, τὰν δὲ γᾶν περιφερέσθαι περὶ τὸν ἅλιον κατὰ κύκλου περιφέρειαν, ὅς ἐστιν ἐν μέσῳ τῷ δρόμῳ κείμενος, τὰν δὲ τῶν ἀπλανέων ἄστρων σφαῖραν περὶ τὸ αὐτὸ κἐντρον²⁵ τῷ ἁλίῳ κειμέναν τῷ μεγέθει ταλικαύταν εἶμεν, ὥστε τὸν κύκλον, καθ’ ὃν τὰν γᾶν ὑποτιθέται περιφερέσθαι, τοιαύταν ἔχειν ἀναλογίαν ποτὶ τὰν τῶν ἀπλανέων ἀποστασίαν, οἵαν ἔχει τὸ κέντρον τᾶς σφαίρας ποτὶ τὰν επιφάνειαν." Heath mentions a proposal of Theodor Bergk that the word "δρόμῳ" ("orbit") may originally have been "ὀυρανῷ" ("heaven", thus correcting a grammatical incongruity) so that instead of "[the sun] lying in the middle of the orbit" we would have "[the circle] lying in the middle of the heaven".</ref>{{#if:|{{#if:|}}
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Aristarchus knew that a moving-earth model would imply that the stars should exhibit parallax (that is, a movement of the stars relative to each other as the Earth moves around the Sun). However, since stellar parallax is only detectable with telescopes, it was not observed at the time. Aristarchus reconciled this issue by postulating that the stars were other suns that are very far away,<ref>{{#invoke:citation/CS1|citation |CitationClass=web }}</ref> far enough that the parallax was not observable. This implied a universe much larger than had been believed.

It is a common misconception that the heliocentric view was considered sacrilegious by the contemporaries of Aristarchus.<ref name="RussoCleanthes"/> Lucio Russo traces this to Gilles Ménage's printing of a passage from Plutarch's On the Apparent Face in the Orb of the Moon, in which Aristarchus jokes with Cleanthes, who is head of the Stoics, a sun worshipper, and opposed to heliocentrism.<ref name="RussoCleanthes">Template:Cite book; Template:Cite journal</ref> In the manuscript of Plutarch's text, Aristarchus says Cleanthes should be charged with impiety.<ref name="RussoCleanthes"/> Ménage's version, published shortly after the trials of Galileo and Giordano Bruno, transposes an accusative and nominative so that it is Aristarchus who is purported to be impious.<ref name="RussoCleanthes"/> The resulting misconception of an isolated and persecuted Aristarchus is still promulgated.<ref name="RussoCleanthes"/><ref>{{#invoke:citation/CS1|citation |CitationClass=web }}</ref>

According to Plutarch, while Aristarchus postulated heliocentrism only as a hypothesis, Seleucus of Seleucia, a Hellenistic astronomer who lived a century after Aristarchus, maintained it as a definite opinion and gave a demonstration of it,<ref>Plutarch, Platonicae quaestiones, VIII, i</ref> but no full record of the demonstration has been found. In his Naturalis Historia, Pliny the Elder later wondered whether errors in the predictions about the heavens could be attributed to a displacement of the Earth from its central position.<ref>Template:Cite book</ref> Pliny<ref>Naturalis historia, II, 70</ref> and Seneca<ref>Naturales quaestiones, VII, xxv, 6–7</ref> referred to the retrograde motion of some planets as an apparent (unreal) phenomenon, which is an implication of heliocentrism rather than geocentrism. Still, no stellar parallax was observed, and Plato, Aristotle, and Ptolemy preferred the geocentric model that was believed throughout the Middle Ages.

The heliocentric theory was revived by Copernicus,<ref name="Angelo2014">Template:Cite book</ref> after which Johannes Kepler described planetary motions with greater accuracy with his three laws. Isaac Newton later gave a theoretical explanation based on laws of gravitational attraction and dynamics.

After realizing that the Sun was much larger than the Earth and the other planets, Aristarchus concluded that planets revolved around the Sun.

Distance to the SunEdit

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File:Aristarchus working.jpg

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

The only known work attributed to Aristarchus, On the Sizes and Distances of the Sun and Moon, is based on a geocentric worldview. Historically, it has been read as stating that the angle subtended by the Sun's diameter is two degrees, but Archimedes states in The Sand Reckoner that Aristarchus had a value of half a degree, which is much closer to the average value of 32' or 0.53 degrees. The discrepancy may come from a misinterpretation of which unit of measure was meant by a Greek term in the text of Aristarchus.<ref>Template:Cite journal</ref>

Aristarchus claimed that at half moon (first or last quarter moon), the angle between the Sun and Moon was 87°.<ref>Greek Mathematical Works, Loeb Classical Library, Harvard University, 1939–1941, edited by Ivor Thomas, volume 2 (1941), pp. 6–7</ref> Using correct geometry, but the insufficiently accurate 87° datum, Aristarchus concluded that the Sun was between 18 and 20 times farther away from the Earth than the Moon.<ref>A video on reconstruction of Aristarchus' method, in Turkish without subtitles.</ref> (The correct value of this angle is close to 89° 50', and the Sun's distance is approximately 400 times that of the Moon.) The implicit inaccurate solar parallax of slightly under three degrees was used by astronomers up to and including Tycho Brahe, c. 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.<ref>Template:Cite book</ref>

Size of the Moon and SunEdit

In On the Sizes and Distances of the Sun and Moon, Aristarchus discusses the size of the Moon and Sun in relation to the Earth. In order to achieve these measurements and subsequent calculations, he used several key notes made while observing a lunar eclipse.<ref name=":0">Template:Cite journal</ref> The first of these consisted of the time that it took for the Earth's shadow to fully encompass the Moon, along with how long the Moon remained within the shadow. This was used to estimate the angular radius of the shadow.<ref>Template:Cite journal</ref> From there, using the width of the cone that was created by the shadow in relation to the Moon, he determined that it was twice the diameter of the Moon at the full, non-central eclipse. In addition to this, Aristarchus estimated that the length of the shadow extended around 2.4 times the distance of the Moon from the Earth.<ref name=":0" />

File:Aristarchus and Herodotus craters Apollo 15.jpg

Aristarchus (center) and Herodotus (right), from Apollo 15, NASA photograph

Using these calculations, along with his estimated distances of the Sun from the Earth and Moon from the Earth, he created a triangle. Employing geometry similar to that he had already used for the distances, he was able to determine that the diameter of the Moon is roughly one-third of the Earth's diameter. In order to estimate the size of the Sun, Aristarchus considered the proportion of the Sun's distance to Earth in comparison to the Moon's distance from Earth, which was found to be roughly 18 to 20 times the length. Therefore, the size of the Sun is around 19 times wider than the Moon, making it approximately six times wider than the Earth's diameter.<ref name=":0" />