Cleomedes

Cleomedes was a Greek astronomer renowned for his treatise *On the Circular Motions of the Celestial Bodies*, also known as *The Heavens*. While his exact birth and death dates remain unknown, historical estimates place his writing period between the mid-1st century BC and 400 AD. His work extensively references earlier scholars like Posidonius, Aristotle, Pytheas, Aratus, Eratosthenes, and Hipparchus, but notably omits Ptolemy, leading to debated dating. Modern analysis by Otto Neugebauer suggests a date around 371 AD, though this is also contested due to Cleomedes' observational errors. The book itself is a foundational astronomy textbook, notable for its philosophical as well as scientific aims, including criticism of Epicurean ideas. Its primary value lies in preserving significant portions of Posidonius's lost astronomical writings. Cleomedes accurately discussed lunar eclipses and supported the idea of a spherical Earth based on lunar shadows. He also presciently noted that stars could be larger than the Sun and that Earth might appear small from a solar perspective. His work is the source for the account of Eratosthenes' measurement of Earth's circumference, considered a significant ancient astronomical achievement. Cleomedes is also credited with the earliest clear explanation of the Moon Illusion, attributing it to Posidonius. As a follower of Posidonius, he explored basic optical properties like refraction and suggested atmospheric refraction allows visibility of the Sun below the horizon. Cleomedes is memorialized by the lunar crater named after him.