Aristarchus of Samos
The earth goes around the sun
Aristarchus figured out that the earth went around the sun by looking at the shadow of the earth on the moon during an eclipse of the moon (now that Thales had shown how to predict eclipses and Anaxagoras had shown how eclipses worked). He used Euclid’s geometric theories, and his own, to do his calculations.
Some scientists at the time thought this was ridiculous, and instead followed Aristotle’s idea that the sun went around the earth. But others followed Aristarchus. First Archimedes, and then al-Ghazali and other Islamic astronomers took Aristarchus (arr-iss-TAR-kuss) seriously.
The stars must be very far away
Aristarchus also used the work of Eratosthenes to figure out the size of the moon, and to calculate that the sun had to be a lot bigger than either the earth or the moon (though he didn’t get the sizes right because he didn’t have a good telescope).
Aristarchus also realized for the first time that the stars must be very far away, much farther than the sun or the moon, but again, he couldn’t prove it without a good telescope.
Greek and Roman Science, by Don Nardo (1998). Nardo has written a lot of good books about the ancient world for kids; this one is no exception.
Ancient Science: 40 Time-Traveling, World-Exploring, History-Making Activities , by Jim Wiese (2003). Activities, as the title says – how to make your own sundial, and so on. The author is a science teacher.
Early Greek Science: Thales to Aristotle, by Geoffrey Lloyd (1974).
History of Greek Mathematics: From Aristarchus to Diophantus, by Thomas L. Heath (1921, reprinted 1981). A lot of Euclid, but also describes who the other major Greek mathematicians were and what they did.
Episodes from the Early History of Mathematics, by Asger Aaboe (1997).