Sky Lights

The time has come for purists to celebrate the actual turning of the millennium, which arrives at the end of this month, 2,000 years after A.D. 1. Staring up at the dark, moonless sky this New Year's Eve, one might be tempted to imagine night travelers 10 or 20 centuries ago looking up at the very same sky and seeing the same stars. But that would be wrong. The night sky this month is quite different from the scene 1,000 years past. Tiny celestial motions, too small to notice from day to day, transform the heavens over time.