Cleomedes

Cleomedes () was a Greek astronomer who is known chiefly for his book On the Circular Motions of the Celestial Bodies (Κυκλικὴ θεωρία μετεώρων), also known as The Heavens ().

==Placing his work chronologically== His birth and death dates are not known—historians have suggested that he wrote his work sometime between the mid-1st century BC and 400 AD. The earlier estimates rely on the fact that Cleomedes refers extensively in his writing to the work of mathematician and astronomer Posidonius of Rhodes (c. 135 – c. 51 BC), and yet seemingly not at all to the work of Ptolemy (c. 100 – c. 170 AD). Cleomedes also refers to Aristotle (384–322 BC), Pytheas of Massalia (310–306 BC), Aratus (c. 315/310 BC – 240 BC), Eratosthenes (276–195 BC), and Hipparchus (c. 190 – c. 120 BC). These conclusions have been challenged on the grounds that Cleomedes' work was in relatively elementary astronomy, and that reference to Ptolemy would not necessarily be expected. The 20th century mathematician Otto Neugebauer, however, looked closely at the astronomical observations made by Cleomedes, and concluded that a date of 371 AD (±50 years) better explains what is found there. Neugebauer's estimate has been challenged on the grounds that Cleomedes makes observational errors with enough frequency that there is difficulty in deciding which observations to trust for the purpose of dating his work.