Some of their astronomical triumphs are:
• Aristotle (384-322 B.C.):
  • spherical Earth <===> changing appearance of the sky with latitude on the Earth and the curved shape of the Earth's shadow on the Moon during lunar eclipses,
    

• Aristarchus (310-230 B.C.):
  • Helio-centric model for the Solar System (not believed because parallax was not observed). We note that Aristarchus had a physical reason for believing in a helio-centric theory. (Copernicus 1700 years later was driven more by aesthetic arguments.)

    Argument: Aristarchus first showed that the Sun was much larger than the Earth. Aristarchus then concluded that it was therefore much more likely that the Sun lived at the center of the Solar System rather than the Earth!

• Erastosthenes (273-? B.C.)
  Circumference of the Earth: As you in move in latitude on the Earth, the appearance of the sky changes. Erastosthenes took advantage of this fact to infer the size of the Earth.

On the summer solstice, the Sun casts no shadow at "noon" at Syrene (latitude ~ 23.5 degrees). On the solstice, the Sun passes directly overhead at noon in Syrene. At Alexandria, the Sun casts a short northward shadow at "noon" suggesting that the Sun is passing south of the zenith at noon. The length of the shadow suggested that the angle the Sun made with the vertical was 7 degrees. Hmmmm, this means that Alexandria and Syrene are separated by 7 degrees in latitude or that they are 7/360-ths of the circumference of the Earth apart. Since the distance from Syrene to Alexandria could be measured, Erastosthenes deduced the circumference of the Earth. He came within ~2 % of the current estimate!