There is no mistaking Jupiter in our summer evening skies. It pops out even before the end of evening twilight in the low south-southeast. It’s by far the brightest star-like object in the evening heavens.
Jupiter has a buddy in the sky this year. Just to the left of the behemoth of our solar system is Saturn, the second biggest planet in our local planet family. Saturn and Jupiter are less than 10 degrees apart, less than the width of your fist held at arm’s length. As summer turns to fall, the two planets will get even closer to each other, setting up for a tremendous celestial hugging toward the end of the year.
Even though Jupiter is nearly 405 million miles away, it’s a real treat to check it out through even a small telescope, and to 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 and very hot core. As long as it’s close by, feast your eyes on Saturn and its gorgeous ring system.
Sliding your telescope back to Jupiter you’ll see what Galileo Galilei saw in the 1600s, something that eventually got him in a lot of trouble. On most evenings there are little diamond-like “stars” on both sides of Jupiter. These are Jupiter’s brightest and biggest 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. On the diagram you can see the alignment of Jupiter’s moons from Sunday through next Friday. There’s a great website from Shallow Sky that will help you keep up with Jupiter’s moons from night to night: shallowsky.com/jupiter/. There are also great apps like “Jupiter’s Moons” and Sky Guide.
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 know the nature of either Jupiter or the moons, but he concluded that the moons were definitely orbiting Jupiter. 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 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 arranged to have his theory published 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. Hundreds of years later in 1992, 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. Recently water vapor plumes have been observed emerging from Europa. It’s almost certain that beneath the ice is an ocean of liquid water that may harbor some kind of life. You may wonder how it could be warm enough almost half a billion miles from the sun for liquid water to even be on Europa. The answer is Jupiter’s immense gravitational field that exerts extremely strong tidal forces on Europa, constantly stretching and compressing the little moon. The constant stress on Europa builds up heat in its core, enough for liquid water. Anyone want to go ice fishing on Europa? NASA’s Europa Clipper spacecraft will certainly be fishing for more information, and will try to help astronomers determine if the waters of Europa could support life. It won’t drill holes into the ice, but will orbit the moon, making many really close passes and electronically relaying data back to Earth. The date hasn’t been set, but the Europa Clipper will launch to Jupiter later this decade. Stay tuned!
Those same strong tidal forces raise holy hannah 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.
Enjoy Jupiter’s dancing moons as you keep in mind some of the bizarre happenings on Galileo’s little friends.
CELESTIAL HAPPENING THIS WEEK: This coming Friday and Saturday evenings there’s going to be a nice conjunction or celestial hugging between the moon, Jupiter and Saturn. The near-full gibbous moon is going to be passing below the super bright planets Jupiter and Saturn in the early evening low south-southeastern sky.
Mike Lynch is an amateur astronomer and retired 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 adventurepublications.net. Mike is also available for private star parties. You can contact him at firstname.lastname@example.org.
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