Turn, turn, turn: in addition to its daily spin and its annual trip around the Sun, the Earth wobbles—affecting the seasons, the "north star," and human history

One and a half millennia later, at the time Homer composed the Odyssey, precession had left Thuban relatively useless as a north star. Homer's wandering hero Odysseus had to do his best with the Big Dipper--the seven bright stars of the constellation Ursa Major, or big bear, which then, as now, lay relatively close to the north celestial pole: "For so Kalypso, bright among goddesses, had told him to make his way over the sea, keeping the Bear on his left hand." In fact, the constellation would have moved around the sky quite a bit during the night, making Odysseus's navigational task considerably more difficult than Kalypso's directions imply. Still, the Big Dipper was a rough-and-ready indicator of the way north [see illustration above].

The discovery of precession, like Homer's great poem, was a signal achievement of ancient Greek culture. The Greek astronomer Hipparchus, who lived during the second century B.C., is justly famous for being the first to note its effects. Hipparchus made his breakthrough not by observing the position of the north celestial pole, but rather by noting some of the other changes caused by precession. High among them are the times of the year when the Sun reaches particular points on the sky, as it seems to move among the constellations, blocking some of them from view. (Astronomers now realize, of course, that it is the Earth that moves.)

Even as the orientation of the Earth's rotation axis wobbles, or precesses, the Sun continues to take its yearly lap around the sky, along the path called the ecliptic. Although ancient astronomers could not see the stars that happened to lie behind the Sun at various times of year, their excellent record keeping enabled them to accurately reconstruct which constellations provided a "house" for the Sun at any given moment.

There were twelve such houses, which match the familiar constellations, or signs, of the zodiac; together they form a band around the sky that includes the ecliptic. In the zodiacal system for keeping track of the year, created by astronomers in ancient Mesopotamia, each time the Sun entered a new house heralded the beginning of a new month. Every new year, moreover, began on the spring equinox when, as the Mesopotamians had determined, the Sun blocked the constellation Aries from sight.

But Hipparchus noted that something had happened during the two millennia since the Mesopotamian system had been codified: the Sun no longer occupied its specified position on the first day of spring. Instead, he determined, the Sun was reaching its marks along the ecliptic progressively earlier, by one day every seventy-two years. Because precession changes the times of the year of the spring and fall equinoxes--which we also measure by the visible seasonal changes on Earth--the effect acquired its full title: "precession of the equinoxes." If you could wait for 365 times seventy-two years--approximately 26,000 years--you would find that the equinoxes take place once again at their original times of the year, because one full cycle of precession had finished. Nowadays on the spring equinox, the Sun is near the first point in the constellation Aquarius, which leads some astrologers to refer to our epoch as the "Age of Aquarius."