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.
At midnight on December 31, blue-white Sirius, the brightest star at night, will dominate the southern sky, and yellowish Capella will be at or near the zenith. One millennium ago, Sirius was much lower, and Capella was nowhere near the overhead point. A few more 1,000-year-jumps back in time and Sirius was completely out of sight— the loveliest star never rose above the horizon in what is now the United States. On the other hand, the now-hidden Southern Cross could have been seen from Maine. A few millennia before that, Scorpius, which scrapes our night horizon, soared nearly overhead, illuminating the winter landscapes of the first agricultural communities.
The cause of these changes lies not in the stars but in ourselves: Earth's axis wobbles like the shaft of a spinning top. That sweeping motion shifts not only the sky but also the seasons. These days, the northern hemisphere is angled sunward on July 4, when our planet's oval orbit takes us farthest from our star. The extra distance makes our summers a little cooler and more tolerable than they would be otherwise. Conversely, winters in the northern hemisphere are a bit warmer because that's when Earth huddles nearest to the sun.
A dozen millennia back, the situation was reversed. The United States, Europe, and Canada angled toward the sun just as it was closest to Earth. The resulting 7 percent increase in summer solar intensity— and the equal reduction during winter— made for significantly harsher seasons.
We're now at the maximally mild point in Earth's 26,000-year axial wobble. For the next dozen millennia, winters will grow colder, summers more torrid. Sirius will dip lower until once again it vanishes. The nearest star, brilliant Alpha Centauri, will become visible throughout the United States. Long before then, Polaris will no longer be the North Star. The baton will pass to a series of faint stars and then, around A.D. 9000, to bright Deneb.
Not all the sky's transformations are caused by earthly motions. Some are genuinely celestial. Every 1,000 years, Sirius drifts south in the sky by nearly the diameter of the full moon, while Alpha Centauri slides more than two moon-widths to the west. Every few hundred years, on average, we see a spectacular supernova explosion. There is a small chance that Betelgeuse, the bright red star in the shoulder of Orion, will blow itself to bits before A.D. 3000.
Yet one trustworthy companion will keep us company through the ages. Saturn shines brightly in each and every new-millennium sky because its orbital period of 291/2 years divides almost evenly into 1,000. The ringed planet will continue to blaze for millennial celebrations well into the Deneb era— as it has done since before humans discovered how to create fire.
A simple, graphic explanation of how Earth's wobble affects the intensity of the seasons exists at www.cotf.edu/ete/modules/msese/dinosaurflr/wobble.html, made in conjunction with NASA's Classroom of the Future.
