Sky Watch: Jupiter is back

I miss Jupiter.

That may seem like a corny thing to say, but when one enjoys viewing a planet so much, it's really kind of sad to see it leave our evening sky. Yet, since Earth and its companion worlds orbit the sun, that's just what planets do each year, so one must get used to it.

But the great planet is back in the early evening sky now, and it's going to be a terrific season of Jupiter-watching.

Jupiter will reach its opposition point on the evening of Dec. 2. The word "opposition" is one that astronomers throw around without realizing that most beginners don't have a clue what it means, but it's really quite simple.

When a planet lies at opposition, it appears in our sky opposite the sun. In other words, the Earth lies between the sun and the planet. It's the time when -- for a couple of weeks -- the planet rises in the east around sunset and is visible all night long. It's also when the planet reaches its closest point to the Earth and, therefore, appears at its largest and brightest.

So Jupiter, which is always impressive to view through a small telescope, will be especially stunning over the next month or so, and there's no better time for Jupiter-gazing than right now.

Here's a world that's 11 times the diameter of Earth, yet it rotates on its axis once every ten hours or so. This means that its Earth-facing side changes completely in just five hours and, with patience, sky watchers with a small telescope can easily watch its pastel cloud bands and Great Red Spot spin around in just one evening of stargazing.

Equally amazing is that Jupiter is composed entirely of gas; there is absolutely no surface on which to stand. Hypothetical astronauts trying to "land" on the planet would just sink deeper and deeper into its murky atmosphere until they'd be crushed beyond recognition by its tremendous weight.

If that's not remarkable enough, consider this: Jupiter is orbited by a family of more than five dozen moons, and what I enjoy most is keeping up with the antics of its four largest: Io, Europa, Ganymede and Callisto.

These moons do-si-do around the Jovian globe from night to night -- sometimes vanishing behind the giant planet or even casting their shadows onto Jupiter's cloud tops -- and provide endless entertainment for the backyard astronomer.

Half the fun of watching these moons is knowing their names and their characteristics. You can identify them by visiting http://www.shallowsky.com/jupiter/, where you can type in the date and time and see a graphic representation of their positions, along with that of the Great Red Spot. And to learn more about these incredible moons and their parent planet, be sure to check out nineplanets.org/jupiter.html.

Now that Jupiter has returned to the early evening sky, try aiming your telescope in its direction, or contact your local astronomy club or science museum to learn when they'll be hosting their free "star party" so you can get a close-up look at this exciting giant planet.

Yes sir, it's going to be another great season of Jupiter-watching!