Starwatch: Get all warm and cozy with a celestial hug

Chart of Jupiter's moons the next seven nights. For Jan. 18, 2013, Starwatch column.

Jupiter, the goliath planet of our solar system, is continuing to put on quite a show. Right now, it's high and bright in the early-evening southeastern sky, not all that far from the overhead zenith. You'll have absolutely no problem finding it, since it's the brightest star-like object in our sky right now. As you gaze upon its brilliance, Jupiter is about 413 million miles away with a diameter of 88,000 miles, more than ten times the girth of our world. In fact, if Jupiter were hollow you could fill it with well over a 1,000 Earths!

The Jupiter show will take on a vivid partner early this week — Earth's moon. On Sunday night, the waxing gibbous football-shaped moon will be just a little to the left of Jupiter, but on Monday night, they will be in a very tight celestial hug that you don't want to miss — if the clouds stay away that is. The moon will be just one degree to the left of Jupiter. That's only the width of your forefinger at arm's length.

Now, the moon and Jupiter aren't anywhere close to each other physically, but their paths among the stars are nearly in the same plane. The moon rips among the backdrop of stars as it orbits our Earth in just over 27.3 days. It takes Jupiter 12 years to make the same circuit among the stars as it obediently circles the sun every dozen years.

Earth's moon isn't Jupiter's only company this week. Jupiter also has its own moons that circle the behemoth planet all the time. At last count, Jupiter has well over 60 moons. Four of its largest can be seen on either side of the planet with a small telescope or even a decent pair of binoculars. They look like stars. With a small telescope you should also be able to see at least some of the methane and ammonia cloud bands across Jupiter's face.

The number of moons you see and where they are with respect to Jupiter depend on where they are in their 2- to 17-day orbits around the big guy. The moons pass behind Jupiter on a regular basis and are lost from our sight, and they also pass in front of Jupiter and get camouflaged against the planet's clouds. If viewing conditions are right and your scope is big enough you can actually see little dots on the face of Jupiter that are the passing shadows of the moons.


A long time ago, Galileo Galilie (1564-1642) also spotted Jupiter's largest moons. They played a huge role in helping Galileo prove that the sun, and not Earth, is the center of our solar system. In fact, these four moons are referred to as "Galilean" moons.

Almost 410 years later, Galileo again made new discoveries about Jupiter and its moons — only this time it was the Galileo space probe, now winding down after six years of exploration. Before Galileo, the Pioneer and Voyager probes gathered pictures and data of the Jovian system. Because of these missions and other observations, we know a lot more about Jupiter's moons, especially the four big Galilean moons. The two outer moons, Callisto and Ganymede, are old, crater-covered moons, about four and half billion years young and not all that interesting except for the fact that Ganymede is the largest moon in our solar system, more than 3,400 miles in diameter.

On the other hand, Jupiter's closest moons, Io and Europa, are anything but boring. Io, the closest moon, has numerous active sulfuric volcanoes and constant lava flows because of the tremendous tidal stress from its mother planet. When the first color photographs of Io came back, astronomers dubbed Io the "pizza planet" because it was various shades of orange and red from all of the volcanism on its surface.

The second closest moon, Europa, may have a slushy ocean under a thin layer of ice. It could be the only other place in the solar system besides Earth that has liquid water — and where there's water, possibly life? Stay tuned.

The diagram shows the positions of Jupiter's moons relative to the planet over the next seven nights. A good website to keep up with the whirling Galilean moons is at Sky and Telescope Magazine:

There are also many apps that keep up with Jupiter's moons on both Android and iPhones.

What To Read Next
Get Local