**In the third century BCE, the Greek astronomers first attempted to estimate cosmic scales. From approximating the size of the moon to understanding orbital motion, ancient Greek and Hellenistic astronomers paved the way for modern astronomy.**

Aristarchus of Samos (310–230 BCE) was a Greek astronomer and mathematician. In this work, he talks about the relation between the sun, moon, and Earth. By observing different situations (ie. half-moon, lunar eclipse) he attempted to figure out the relative distance of the sun and moon from the Earth and the radii of each, all in terms of ratios to one another. Using light transmission, angles subtended, and shadow, he proved his propositions with geometric and trigonometric methods.

From there he reached more remarkable conclusions. Aristarchus proposed that all planets circle around the sun, the first known heliocentric model. Aristarchus was also the first to propose the idea of a solar system. These ideas were rejected at the time, lost, and rediscovered 2000 years later!

*Almagest*, written in about 150 AD by Egyptian-Greek astronomer Claudius Ptolemy, was the standard textbook of astronomy for more than a thousand years. Ptolemy describes the motions of the heavenly bodies, relying on principles of geometrical astronomy that were established centuries earlier by the Assyro-Babylonians and ancient Greeks, with the most basic principle being the idea that they move in a uniform circular motion and the speed and direction of their movement remains the same.

Further, Ptolemy argued that a stationary Earth was at the center of the universe, and is circled by the sun, the moon, and all the other known planets of the time. In doing so, he rejected the hypothesis of Aristarchus of Samos. His geocentric model, which is popularly known as the Ptolemaic system, had been the accepted cosmological model, however, in time, manuscripts began to appear that questioned several of its precepts.

Works cited:

“Aristarchus of Samos (310-230 BC) | High Altitude Observatory.” *Www2.Hao.ucar.edu*, www2.hao.ucar.edu/Education/FamousSolarPhysicists/aristarchus-samos.

“Ptolemy - Claudius Ptolemy Geocentric Model - World Systems - Dr Robert A. Hatch.” *Users.clas.ufl.edu*, users.clas.ufl.edu/ufhatch/pages/03-Sci-Rev/SCI-REV-Home/resource-ref-read/chief-systems/08-0PTOL3-WSYS.html.

“Ptolemy.” *Uoregon.edu*, 2019, abyss.uoregon.edu/~js/glossary/ptolemy.html.

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