Galileo used to view them

There’s no mistaking Jupiter in the Butler sky this spring. It pops out even before the end of evening twilight in the high southwestern sky, and it is by far the brightest starlike object in that part of the sky.

Tonight, it’s extra easy to locate as the waxing gibbous moon will be parked just to the lower left of Jupiter.

Even though it’s nearly 475 million miles away, you can look through even a small telescope and see at least some of the cloud bands that circle the gargantuan 88,000-mile wide planet. These cloud bands are made of sulfur, methane and other gases. Underneath the cloud bands, Jupiter is basically a giant ball of hydrogen gas with a solid core.

With your telescope you can also see something Galileo Galilei saw in the 1600s, something that eventually got him in a lot of trouble.

There are little “stars” that show up in a line on either side of Jupiter. These are actually Jupiter’s brightest moons. As they orbit Jupiter in periods of 2 to 17 days they constantly change their alignment on either side of the planet.

Some nights you don’t see all four of the moons, because one or more of them may be either behind or in front of Jupiter, lost in the backdrop of the planet’s glow.

You can see the alignment of Jupiter’s moons from Sunday through Friday on the diagram. The website www.shallowsky.com/jupiter will also help you keep up with Jupiter’s moons from night to night.

These four moons are referred to as the Galilean moons because Galileo Galilei watched them as often as he could with his small telescope in the early 1600s. He didn’t quite know the nature of either Jupiter or the moons, but he concluded the moons were definitely orbiting Jupiter.

The century prior, the famous Polish astronomer Nicolas Copernicus proposed the sun, and not Earth, was the center of the universe. Back then, the government and Catholic Church treated that as sheer heresy. To suggest Earth was not the center of the universe got you in big time trouble, so much so that Copernicus didn’t publish his theory until the day he died.

Galileo was privately a big fan of the Copernican theory. When he observed Jupiter’s moons circling the planet, he reasoned that if Jupiter could be the center of its own little universe, why couldn’t the sun be an astronomical hub?

He published his observations and theories and was convicted by the church and sentenced to house arrest for the rest of his life. It wasn’t until 1992 that Pope John Paul II finally pardoned Galileo — better late than never.

Thanks to the fleet of robotic spacecraft that have visited or passed by Jupiter in the last 30 years, we now know a lot more about the Galilean moons. The two outer moons, Callisto and Ganymede, are the largest of Jupiter’s moons, with diameters of around 3,000 and 3,300 miles respectively, both way bigger than our own moon.

Callisto is the most heavily cratered object we know of in the solar system, just over a million miles from Jupiter.

Ganymede, about 700,000 miles from the big mother planet, is the largest moon in the solar system, even larger than the planet Mercury. It has mountains, valleys, and even lava flows, but not much of an atmosphere.

The two most interesting moons by far are the inner Galilean moons, Europa and Io.

The surface of Europa is a giant sheet of cracked ice about 3 miles thick. In fact, water vapor plumes have recently been observed emerging from Europa. It’s almost certain that beneath the ice is an ocean of liquid water that conceivably harbors some kind of life.

You may wonder, and rightly so, how could it be warm enough almost half a billion miles from the sun for liquid water on Europa?

The answer is Jupiter’s immense gravitational field that exerts extremely strong tidal forces on Europa, constantly stretching and compressing the planet. That constant stress on Europa builds up heat in its core, enough heat for liquid water. Anyone want to go ice fishing on Europa?

Those same strong tidal forces literally raise hell on Io, Jupiter’s closest moon, only 260,000 miles from the giant. It’s only about 1,100 miles in diameter, but it’s the most volcanically active body in the solar system.

Because of the constant volcanic eruptions, Io’s surface is a chaotic mix of reds, oranges, yellows and grays. In fact, many astronomers refer to Io as the “pizza planet.”

Many of these eruptions have been caught by the cameras of space probes, spewing almost 200 miles above the surface. Recently, the New Horizon probe on the way to Pluto imaged some fantastic eruptions on Io.

Enjoy Jupiter’s dancing moons as you keep in mind some of the bizarre happenings on Galileo’s little friends.

Mike Lynch is an amateur astronomer and professional broadcast meteorologist for WCCO Radio in Minneapolis/St. Paul and is author of the book, “Stars, a Month by Month Tour of the Constellations” published by Adventure Publications and available at bookstores and at www.adventurepublications.net.