Astronomy: Peter Apian

Astronomy was a discipline of mathematics, not Aristotelian physics. Ancient and medieval astronomers knew that the Earth was round, despite (Figure 1) , a famous woodcut that actually originates from a 19th-century source (more on this non-medieval woodcut).

Peter Apian’s Cosmographia (Figure 2) shows the map of the universe that ordinary students took for granted (Figure 3) . A central spherical Earth is surrounded by the Moon embedded within a solid sphere carrying it around the Earth once a month. The solid sphere explains why the same side of the Moon always faces the Earth. Another solid sphere contains the fixed stars, which rotate around the Earth once each day. The solid sphere explains why the stars appear fixed in constellations during their daily motion. It also suggests that the stars are more or less the same distance away from the Earth. If the idea of spheres works so well for the Moon and stars, why not for the other planets in between, including the Sun? Extra spheres could be added for long-term periodical phenomena such as precession.

Simple woodcut illustrations abound in Apian’s textbook. (Figure 4) shows one of Aristotle’s arguments for the sphericity of the Earth: during a lunar eclipse, the Earth’s shadow on the Moon is always curved. If the Earth were any other shape, some time or other its shadow would be angular. Therefore, one didn’t have to circumnavigate the globe to know that the Earth is round, and the mistaken idea that Columbus faced opposition from flat-earthers is a modern myth.

Exhibit credit: Kerry Magruder, with the assistance of , Marilyn B. Ogilvie, Duane H. D. Roller.