Correspondence

摘要

The substantial achievements in astronomy that Nidhal Guessoum refers to occurred earlier than the golden age' of Arab astronomy from the ninth to the sixteenth century ad {Nature 498,161-164; 2013). Astronomy developed between the fourth century bc and the first century ad, but especially in the third century bc. It matured from tables of observations, from which only a few general patterns were recognized (the saros eclipse cycles, for example), into a sound understanding of the Solar System. This included good estimates for the size of Earth and the sizes of the Sun and the Moon, as well as their distances, and the discovery of the precession of the equinoxes. More importantly, a majestic mathematical construction allowed the prediction of the positions of all of the major bodies visible in the sky, with a precision close to the best available with observation (10 minutes of arc).