North Stars 
The North Star is one of the most important beacons in the night sky. It serves as a compass, pointing the way due north. And it also serves as the hub of the night sky -- all the other stars appear to circle around it as Earth turns on its axis.
More than four millennia ago, the architects of ancient Egypt used the North Star to align the pyramids of Giza. And four decades ago, Apollo astronauts used it to help guide them to the Moon.
The problem is, the two groups were guided by different North Stars. The Egyptians used a star called Thuban, in the constellation Draco. And the astronauts used Polaris, at the end of the handle of the Little Dipper.
Thuban didn't explode or fade away -- it's still in plain sight. Instead, Earth's axis turned away from the star -- an effect called precession.
It's caused by the gravitational tug of the Sun and Moon, which cause our planet to wobble like a spinning top. As it wobbles, the axis points toward different stars. Four millennia ago, it aimed toward Thuban. Today, it aims at Polaris.
Over the next century, the pole will actually take slightly better aim at Polaris. Right now, the star is about two-thirds of a degree from the point in space where the axis is aiming. Around the year 2100, it'll be less than half a degree from the true pole.
After that, though, the pole will move away from Polaris. By around the year 4100, it'll take aim at a star in Cepheus. More about that tomorrow.
Script by Damond Benningfield, Copyright 2010