Sky Watch: The Dragon after dark

Wouldn't it be great to have a time machine? Just imagine all the cool things we could do.

We could travel back into time to straighten out all that we fouled up in the past or speed forward to preview exciting advances of the future. We could even peer into the same starry heavens pondered by such luminaries as Aristotle, Copernicus and Galileo.

Well, for the first two I'm afraid you're on your own. As for the last one, I think I can help. In fact, if you'd like to see very same sky viewed by anyone throughout recorded history, all you've got to do is go outside and look up.

That's right. For the most part, the heavens haven't changed much in all that time. Scorpius is still Scorpius; the Dippers are still the Dippers.

There are subtle changes, of course. The moon swings across our nighttime sky every month, changing its phase as it goes. The planets wander gracefully among the fixed stars of the zodiac from year to year.

But there's another change that occurs in our sky -- one that most of us have never noticed. It's what astronomers call precession.

Precession is the wobbling of the Earth's axis -- the same kind of wobbling experienced by a spinning top. But while a top's precession is easy to see, the Earth's requires some 25,800 years to complete just one cycle.

Because of this, the orientation of the heavens tends to shift slightly over time. Precession causes the Earth's axis to point outward in different directions over the millennia. Right now, our planet's north axis aims roughly toward Polaris, making it our North Star. But ages ago this was not the case.

In fact, back when the ancient Egyptians were building their great pyramids, our planet's northern axis pointed not toward Polaris, but more toward the star Thuban, located in the long, sinuous constellation of Draco, the dragon.

Draco is a large and ancient star grouping that appears nightly in our northern sky. It wraps itself around the north celestial pole and remains perpetually above the horizon for much of the Northern Hemisphere.

After dark this week, go outdoors and search for Draco winding its way across the northwestern sky. Look for its long, faint string of stars beginning between the Big Dipper's bowl and the North Star. Then follow it upward until it snakes back down toward Polaris, where it makes another sharp turn and heads upward once again.

At the upper end of its long, dragon-like body lie four stars that form the head of Draco, but modern amateur astronomers know this shape as the lozenge. And back toward the tail end of the dragon, two-thirds of the way between Polaris and the end of the Big Dipper's handle, lies the faint star known as Thuban -- Arabic for dragon.

If we wait patiently for another few millennia, we'll again see Polaris drift away from the north celestial pole, and watch as Thuban takes its place again as the North Star -- a sort of back to the celestial future!

I wonder if the pyramids will still exist then; I wonder if we will.