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Saturn and Spica

October 30, 2011

You don’t need big telescopes or electronic instruments to learn about the universe. Sometimes, you can learn a great deal with not much more than your eyes alone.

Almost 2200 years ago, for example, the Greek astronomer Hipparchus learned that the stars shift position relative to the Sun from year to year. He did so by measuring the position of Spica, the brightest star of Virgo. Spica just peeks into view in the east at dawn tomorrow, a bit to the lower right of the planet Saturn. Binoculars will help you find them through the twilight.

During a lunar eclipse, Hipparchus measured the angle from Spica to the middle of the Moon. And from that, he calculated Spica’s position relative to the Sun. He found that the star had moved about two degrees since another eclipse 150 years earlier. He realized that the entire celestial sphere — the background of fixed stars — rotated with respect to the Sun.

That rotation is known as the precession of the equinoxes. It’s caused not by the stars, but by Earth. Our planet “wobbles” on its axis like a spinning gyroscope that’s running down. As it does so, the stars appear to shift position relative to the Sun. It takes about 26,000 years to complete one full wobble, and have the stars return to their starting positions.

The precession of the equinoxes was an important discovery because it showed that the heavens can change — a discovery made with simple tools and the nimble mind of Hipparchus.


Script by Damond Benningfield, Copyright 2011


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