impressive accomplishments of ancient astronomy). Cleomedes deserves credit for the earliest clear statement of the apparent distance explanation of the Sun Illusion or Moon Illusion. He argued that the sun appeared farther away on the horizon than in the zenith, and therefore larger (since its angular size was constant). He attributed this explanation to Posidonius. Optics As a disciple of Posidonius, Cleomedes noted some elementary qualitative properties of refraction, such as the bending of a ray toward the perpendicular in passing from a less dense to a more dense medium, and suggested that due to atmospheric refraction, the Sun and its rainbow may be visible when the Sun is below the horizon. Legacy Cleomedes is now memorialized by the crater Cleomedes in the northeastern portion of the visible Moon. References Further reading Alan C. Bowen, Robert B. Todd, Cleomedes' Lectures on Astronomy. A Translation of The Heavens with an Introduction and Commentary. University of California Press, 2004. Helen E. Ross, "Cleomedes (c. 1st century AD) on the celestial illusion, atmospheric enlargement, and size-distance invariance". Perception, 2000, 29, 863–871. External links On the Circular Motions of the Celestial Bodies, original text online and biography English translation of the section on Eratosthenes and the size of the earth at Roger Pearse. An article on Cleomedes' description of atmospheric enlargement from Perception (2000) Edicion Bilingue Griego Español Roman-era Greeks Ancient Greek astronomers Ancient Greek mathematicians