Aristarchus of Samos (; , ; ) 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 that the Sun was just another star.

Born in Samos in approximately 310 BC, Aristarchus likely moved to Alexandria and became a student of Strato of Lampsacus, who later became the head of the Peripatetic school in Greece. According to Ptolemy, Aristarchus observed the summer solstice of 280 BC. Vitruvius writes that Aristarchus built two different sundials: one a flat disc; and one hemispherical. Aristarchus estimated the sizes of the Sun and Moon as compared to Earth, and the distances from the Earth to the Sun and to the Moon. His estimate that the Sun was 7 times larger than Earth (it's actually 109 times, in diameter) brought about the further insight that the Sun's greater size made it the most natural central point of the universe, as opposed to Earth.

Aristarchus was influenced by the concept presented by Philolaus of Croton (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.

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.

Heliocentrism

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:

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Aristarchus proposed that the fixed stars were extremely distant, and because ancient cosmology placed them all on a single celestial sphere, the modern concept of stellar parallax did not apply to his model. He placed the stars at a great distance so that their apparent positions relative to each other would remain constant throughout Earth's motion. Aristarchus reconciled this issue by postulating that the stars were other suns that are very far away, In the manuscript of Plutarch's text, Aristarchus says Cleanthes should be charged with impiety.

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, 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. Pliny and Seneca 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, 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 Sun

thumb|right|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.

Aristarchus claimed that at half moon (first or last quarter moon), the angle between the Sun and Moon was 87°. 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. (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.

Similar attempts to estimate celestial distances were later developed in Indian astronomy. Scholars such as Aryabhata and Bhāskara I used mathematical models to explain planetary motion, eclipses, and astronomical measurements. Some modern commentators have also interpreted a verse from the Hanuman Chalisa as describing an approximate distance between the Earth and the Sun, although historians generally regard the text as devotional rather than scientific literature.

Size of the Moon and Sun

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. 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. 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. the minor planet 3999 Aristarchus, and the telescope Aristarchos are named after him.

See also

  • Aristarchus's inequality
  • Eratosthenes (), a Greek mathematician who calculated the circumference of the Earth and also the distance from the Earth to the Sun.
  • Hipparchus (), a Greek mathematician who measured the radii of the Sun and the Moon as well as their distances from the Earth.
  • Posidonius (), a Greek astronomer and mathematician who calculated the circumference of the Earth.

References

Bibliography

Further reading

  • Biography: JRASC, 75 (1981) 29
  • First estimate of the Moon's distance and First estimate of the Sun's distance from educational website From Stargazers to Starships
  • Aristarchus of Samos, The Ancient Copernicus (https://archive.org/details/aristarchusofsam00heatuoft
  • Online Galleries, History of Science Collections, University of Oklahoma Libraries High resolution images of works by Aristarchus of Samos in .jpg and .tiff format.