Jupiter’s dancing moons
There’s no mistaking Jupiter in the sky these Butler winter evenings, rising in the southeast as evening twilight ends. In fact, you should be able to easily see it long before twilight fades. Jupiter is by far the brightest starlike object in the heavens this winter. With even a smaller telescope, you can see some of the cloud bands that circle the gargantuan 88,000-mile-wide planet.
Want to see something Galileo saw, something that got him in a lot of trouble? While you’re enjoying Jupiter through your scope, nearly 398 million miles away right now, look for up to four little stars on either side of Jupiter. These are known appropriately as the Galilean moons. They are four of Jupiter’s 97 known moons, with many more likely. All are gravitationally enslaved to the mighty massive planet. From night to night, the Galilean moons constantly change their alignment with Jupiter as they endlessly dance around the great planet in orbital periods of two to 17 days. Many nights you don’t see all four 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. If you have a powerful enough telescope, you might even see shadows of the moons against the face of Jupiter. There are many ways to keep up with the ever-changing alignment of the moons. One way is Stellarium, a free online or app planetarium program. There’s also great apps like Sky Guide and others.
Galileo watched “his” moons 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 definitely concluded that the moons were orbiting the planet. In the century prior, the famous Polish astronomer Nicolas Copernicus proposed that the sun, and not the Earth, was the center of the universe. Back then the government and the Catholic Church treated that as sheer heresy. To suggest that 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. So 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 half-century or so, we 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 Earth’s moon. Callisto is also one of the most heavily cratered objects 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 three miles thick. Astronomers don’t know for sure, but beneath the ice is a huge saltwater ocean that may contain twice the amount of saltwater as all of Earth’s oceans combined. The water is also warm enough to possibly support life as we know it. As far away as Jupiter and its moons are from the sun, you may wonder how it could be warm enough for liquid water under Europa’s ice. It’s due to the very strong tidal forces of Jupiter, less than 400,000 miles away, that deforms Europa and produces internal friction and heat. Who knows, someday you may want to put your ice fishing house on Europa’s surface. Just make sure you bring a heck of a long auger!
Those same strong tidal forces literally raise hell on Io, Jupiter’s closest moon, only 260,000 miles from the giant planet. 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, Io is referred to as the “pizza planet” by many astronomers. Many of these eruptions, some spewing almost 200 miles above the surface, have been caught easily by the cameras of space probes.
Enjoy Jupiter’s dancing moons and keep in the back of your astronomical minds some of the bizarre happenings on Galileo’s celestial friends!
Mike Lynch is an amateur astronomer and professional broadcast meteorologist for WCCO Radio in Minneapolis/St. Paul. He is also the author of “Stars: a Month by Month Tour of the Constellations,” published by Adventure Publications and available at bookstores and at adventurepublications.net. Contact him at mikewlynch@comcast.net.
