Mooning Around With Jupiter

By Dennis Mammana

April 20, 2017 4 min read

Week of April 23-29, 2017

It was little over four centuries ago that the great Italian astronomer Galileo Galilei first aimed his telescope skyward. It's amazing how many people believe that it was he who invented the telescope, but this was most likely an invention from Holland; in fact, the Dutch were using the device as a military tool decades before Galileo built his own.

Galileo was the first to use a telescope to study the heavens systematically. Over time, his new optical tube revealed to him mountains, valleys and craters on the moon, spots on the sun and the ever-changing phases of Venus. But when he aimed his telescope in the direction of Jupiter, he received the surprise of his life.

Through his eyepiece 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, Galileo looked again, and something was different. The three tiny stars were not where they had appeared the night before — they all lay to the planet's west. If Jupiter had moved past them as it orbited the sun, he reasoned, then it did not behave as astronomers had predicted. He was anxious to see what would happen next.

After a night of clouds he looked again. 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 he 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 Catholic Church had long insisted, 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 get until 2022.

This week, sky watchers can find this brilliant planet midway up in the eastern sky not long after dark.

With even the smallest of telescope, one can make out its 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 their shadows onto the cloud of this magnificent planet and create tiny black dots that drift across its face during the night.

If you don't have your own telescope, contact your local amateur astronomy club, observatory or science museum to learn when they might be hosting their next star party, so you can peek at this exciting world.

You'll really be glad you did!

 View Jupiter's family of moons after dark this week.
View Jupiter's family of moons after dark this week.

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