Greek Astronomy: Sun
At first the Greeks believed that the sun was the god Helios, or Apollo, driving his chariot around and around the earth. In the morning he began driving up in the sky, and then in the evening he drove back down again, and that was the sunset. At night the horses rested under the earth. (Compare to the Egyptian sun god Ra.)
By the Classical period, though, Greek scientists like Thales were beginning to understand that the sun was not a god but a round ball of fire hanging in space. Still they thought that the sun went around the earth instead of the way it really is, that the earth goes around the sun.
In the Hellenistic period, scientists like Eratosthenes, Anaxagoras, and Aristarchus began really taking observations and making measurements, and then they did figure out that the earth went around the sun.
To find out more about Greek astronomy and the sun, check out these books from Amazon.com or from the library:
The Librarian Who Measured the Earth, by Kathryn Lasky (1994). An account of the life and work of Eratosthenes, who figured out the circumference of the earth. Explains how he did it. For kids.
Greek Astronomy, by Thomas Heath (1932). A collection of what ancient Greek writers had to say about astronomy, in their own words, with a long introduction. For adults.
The History & Practice of Ancient Astronomy, by James Evans (1998). Includes both the history, and directions to actually re-do the experiments that ancient Greek astronomers used to figure out their conclusions. For adults.