Sky watch: Jupiter's 'Galilean' moons

It was little over four centuries ago that the great Italian astronomer Galileo Galilei first aimed his telescope skyward.

Galileo was the first to use a telescope to study the heavens systematically. When he aimed his instrument in the direction of Jupiter, Galileo received the surprise of his life. Through his tube, the planet appeared unlike the stars. It was a small, round disk with two bright points of light to its east and one to its west.

The following night, he looked again, but something was different. The three tiny "stars" were not where they appeared the night before; now they all lay to the planet's west. If Jupiter had moved past them as it orbited the sun, Galileo reasoned, then it didn't behave as astronomers had predicted. He was anxious to see what would happen next.

After a night of clouds, he looked again. Now he saw only two "stars," and both were to the planet's east. Was he going mad? Was Jupiter swinging like a pendulum in front of the distant stars? Or was something else happening?

Night after night, Galileo watched the show, never knowing quite what to expect. Then, a few nights later, he no longer saw three stars. Now there were four -- one to the planet's east, the others to its west.

Galileo soon realized that these intriguing lights were not distant stars at all but rather moons that orbited their parent planet. It was this historic observation that showed once and for all that the Earth could not be center of the universe, as the ancient philosophers and the church had long believed, for here were four bodies orbiting another world!

Wouldn't it be cool to repeat this historic observation? You can, and there's no better time than right now, since Jupiter is in a perfect position for viewing and lies as close to Earth as it will until 2022.

This week, look for this magnificent planet low in the eastern sky just after dark. It'll be the brightest object in the heavens after the moon.

With even the smallest of telescopes, you can see Jupiter's cloud systems as pastel brown, tan and white bands, as well as its four "Galilean" moons -- Io, Europa, Ganymede and Callisto -- circling the planet from night to night. And, from time to time, one or two of its moons will cast shadows onto the cloud of this magnificent planet and create tiny black dots that drift across Jupiter's face during the night.