Introduction

Astronomy ranks among the oldest of sciences—hardly surprising, since astronomy played such a central role in the rise of civilization. Before the rise, we were clever nomads coop- erating in small groups simply to exist, constantly searching for sustenance. Afterwards, we formed larger groups with greater organization and specialization, nomadic life being replaced by farmers planting crops and raising herds which could feed more people. Civilization gave more time for specialization and increased efficiency, and eventually culture.

The transition hinged on agriculture and that depended on good timing in planting and harvesting crops, leading to a quest for calendars. People began counting moon phases and seeing how noontime shadow lengths changed throughout the year. Such astronomical awareness and increasingly keen observation were in fact prehistoric, and by the time writing began some 5000 years ago there were horizon calendars such as the famous Stonehenge. This could predict sun positions and eclipses throughout the year. The construction material fit the time—the late Stone Age.

By the time of Aristotle (ca. 384–322 bce) the Greeks had already reasoned that the earth was spherical, since its shadow on the moon during eclipses was circular. (This basic insight was lost by the Middle Ages.) The ancient Greeks went further: about 200 bce, Eratosthenes estimated the earth's radius to be approximately 7000 km, respectably close to its actual radius of 6350 km.