Babylonian astronomers have long been recognized as preeminent in the ancient world. A few thousand years before Copernicus they realized that the earth and the other planets were spherical and that they revolved around the sun. With this knowledge they could accurately predict eclipses of the sun and moon. Many modern scholars assumed that the Babylonians developed their astronomy themselves, to meet the need for accurate calculations for their complex astrology. Surprisingly, newly translated Babylonian texts indicate that positions and motions of the stars and planets were calculated instead according to complex equations inherited from the Sumerian civilization. The Babylonians seem not to have understood the theoretical basis of these formulas, only how to use them. The Sumerians had even more exact knowledge of the solar system and its place in the universe than their Babylonian heirs, whom they predate. Their calendar, devised as early as 3000 B.C., is the model for our calendar today, and they evidently understood a number of more arcane astronomical matters. For example, as the earth spins, it wobbles on its axis; this causes a very gradual change-1 degree every 72 years-affecting which star the north pole points to. The phenomenon is called precession. A Great Year-the time it takes before the north pole points to the same north star again-is 25,920 years, calculated by multiplying the 72 years it takes to move each degree by the 360 degrees in a full circle. The Sumerians understood precession and knew the length of the Great Year-an extraordinary feat, given the lengthy observations involved and the instruments available to them. The Sumerians were also able to measure the distances between stars very precisely. But how would earthbound, pretechnological people learn to do this, and, even more mysterious, why? Such star maps are clearly a necessity for space travelers, but what use could the Sumerian have made of them?