Aristarchus of Samos

An ancient Greek astronomer and mathematician who proposed that Earth rotates on its axis and revolves around the Sun, and estimated the relative sizes and distances of the Sun and Moon.

Ancient Greek astronomy: The study of celestial motions and structures developed by Greek thinkers from the archaic period through late antiquity. Aristarchus worked within a tradition that sought mathematical explanations for the heavens.
Heliocentrism: A family of models that place the Sun at or near the center of a planetary system. Aristarchus is the earliest known Greek proposer of a Sun-centered arrangement for Earth and the planets.
Archimedes: An ancient Greek mathematician and engineer who wrote about mathematics, mechanics, and astronomy. Archimedes is the principal surviving source for Aristarchus’s heliocentric hypothesis.
Aristarchus's heliocentric model: Aristarchus’s proposal that Earth rotates daily and circles the Sun annually, as reported by later authors. No complete treatise explaining this model survives, so its mechanics and planetary arrangement remain unclear.
Pythagoreanism: A philosophical and religious tradition founded around the teachings of Pythagoras and his followers. Some earlier Pythagoreans had imagined Earth moving rather than resting at the cosmos’s center.
Geocentrism: A cosmological model that places Earth at the center of the universe or planetary system. This was the dominant framework against which Aristarchus’s moving Earth stood.
The Sand Reckoner: A work by Archimedes that describes a number system capable of expressing enormous quantities of sand grains. Archimedes reports Aristarchus’s claim that Earth circles the Sun while the stars lie vastly farther away.
Stellar parallax: The apparent annual displacement of nearby stars against distant stars as Earth orbits the Sun. An undetected parallax was used to challenge heliocentrism, prompting the possibility that stars were extremely distant.
Philolaus: A fifth-century BCE Greek philosopher associated with a cosmology in which Earth moves around a central fire. His moving Earth preceded Aristarchus’s heliocentric proposal, though it was not Sun-centered.
Lunar phases: The changing visible shapes of the Moon caused by its changing position relative to Earth and the Sun. Aristarchus used the half-moon geometry to estimate the Sun’s distance relative to the Moon.

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