By Keith Johnson
Have you ever wanted to be Galileo? You can do it tonight!
Go outside after sunset, and look fairly low in the south. If you can see anything in the sky at all, other than the Moon, you'll see Jupiter. Some skygazers claim it's brilliant, largely because it's brighter than any star and most of the other planets. To me it appears like a steady beacon. Planets don't twinkle as much as stars do, though on a night when the air is turbulent, even Jupiter might twinkle just a little bit.
Seeing the largest planet in our solar system with the unaided eye, and knowing what it is, is a satisfying thing. But if you have a pair of binoculars, you are in for a treat.Look at Jupiter through your binoculars. Even a cheap pair will show Jupiter as a tiny disk. This is not the case for any star: they look like points even at high magnification.
Now look a little closer. Do you see four tiny stars lined up on either side of Jupiter? Those are its four large moons. They're named after characters from Roman mythology who were involved with Jupiter: Ganymede, Europa, Io and Callisto. Tonight two of them appear on the left, and one on the right.
But where is the fourth moon? If you're looking tonight, Sept. 7, between sunset and 9:18 p.m., you may only see three dots. That's because one of the moons, Io, is situated in front of Jupiter, and may blend in so well with the disk of the planet that you won't notice it. But if you wait until 9:18, you'll see Io start to slowly drift off the disk to the right. Io moves more quickly than the other three, so you can watch it shift in just a few minutes.
If you have very good binoculars, or if you're using a telescope, you may notice what looks like a dark moon silhouetted against Jupiter to the left. This is actually the shadow of Io being cast on the surface of Jupiter. If you were down on the planet, flying around inside that shadow (you couldn't stand, since there is no solid ground on Jupiter), you'd see a solar eclipse!
If the weather and your schedule allow you to see Jupiter and its family tonight, be sure to try again tomorrow night. You'll find that things look quite different. Three moons will appear on the left, and only one on the right. The following night, Sept. 9, all four will appear on the right side.
Each moon orbits Jupiter at its own speed. Io, the closest, takes less than two days to go around Jupiter, while Callisto, the farthest, requires nearly 17 days. This means their positions will be uniquely different every night.
You've just duplicated an important discovery by Galileo Galilei. And it got him in quite a bit of trouble. The established wisdom of the year 1610 claimed that everything in the heavens was perfect and unchanging, and that all the celestial bodies revolved around the Earth. Galileo's observations of the Jovian moons showed that at least some bodies were revolving around a world that wasn't the Earth. The authorities dismissed his claims as the result of fanciful distortions caused by that weird tube he was looking through. Eventually Galileo was arrested and placed under house arrest for the rest of his life.
Since then, we've found there are quite a few smaller moons in orbit around the giant. Your college astronomy textbook may claim a total of 16 (or 12, if you've been out of college for a long time), but it's out of date. The latest tally stands at 63, which makes Jupiter the current leader, but Saturn is not far behind at 60.
The total count for the solar system is 166, or 169 if you include Pluto's three moons (which you probably shouldn't, since Pluto is not counted as a planet now.) Or perhaps more, if some have been discovered since this article was written!
Most of these moons are tiny and very difficult to see even with large research telescopes. But the four Galilean satellites of Jupiter are easy. Go outside the next clear night and be Galileo for a few minutes. I promise you won't get into trouble!
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Astronomy is an active enterprise, especially at Rowan University. A new public planetarium show at the Edelman Planetarium recounts what's happening in astronomy at Rowan, including a voyage out into space to investigate some fascinating objects being studied by three astronomers at the university. "The Rowan Universe" begins Saturday, Sept. 20, at 7 p.m. For more information, see the planetarium Web site.
Keith Johnson is the director of the Edelman Planetarium at Rowan University. He is happy to answer questions from readers at (856) 256-4389 or at johnsonk@rowan.edu. If he doesn't know the answer, he'll make something up... no, I mean he'll do some research!