Jup-shooting

In fall, some people go hunting for ducks in the bay, or go farther afield to hunt larger animals. But you can hunt much larger game – Jupiter – from your own backyard.

Jupiter is just past its opposition on September 21st, and it’s perfectly placed for observing. This year is an unusually close opposition, because Jupiter is approaching the perihelion of its nearly 12-year orbit, its closest point to the sun. That means that Jupiter is a little bigger, as seen from Earth, than it is at most oppositions.

It won’t actually reach perihelion until early next year, in March 2011. That’s roughly midway between this year’s opposition and next year’s; this year’s is the closer of the two, but not by much, so we’ll get very good Jupiter views both this year and next year.

At this year’s opposition (late September), Jupiter’s disk measured 49.8”; by October 1 it shrinks a little to 49.5”. At next year’s opposition at the end of October 2011, it will measure 49.6”. In a really small opposition, like in 2004, its disk extended only 44.6”.

Of course, these differences aren’t much. For comparison, the Galilean moons range between one and two arcseconds in size. Let’s use Ganymede as a unit of measure, since it’s fairly easy to resolve as a disk in our amateur telescopes. The difference in Jupiter’s size between this year’s opposition and one where Jupiter is at its farthest point from the Sun is about three Ganymedes. The difference between this year’s opposition and next year’s is less than 1/20th of a Ganymede.

So enjoy this year’s big Jupiter! But look forward to next year’s opposition as well, when it will be almost as large and quite a bit higher in the sky, transiting at 64 degrees versus 49 this month.

There will be plenty to see on this big Jupiter. Part of what makes this planet so interesting is the ever-changing weather patterns. You’ll never get bored looking at the same thing with Jupiter! And it’s mind-blowing to think that most of these changes are a lot bigger than our whole planet.

As I write this, Jupiter’s South Equatorial Belt (SEB) is still nearly invisible – it faded mysteriously a few months ago while Jupiter was hidden behind the sun – but there are faint outlines showing where it ought to be. Maybe that’s a sign that it’s darkening again. Should be fascinating to watch and see how long it takes to get back to normal.

The faded SEB should make the Great Red Spot (GRS) look redder and easier to spot (ahem) than it has been in past years. Meanwhile, there’s plenty of detail visible in the North Equatorial Belt (NEB) and Jupiter’s other belts and polar regions.

Double shadow transits have been rare beasts lately. But there’s a nice Ganymede/Europa dual shadow transit the night before Halloween, starting a bit after 9 p.m. on Oct. 30. Too bad it’s not ON Halloween so you could show it to the kids – but the early evening hours of Halloween do offer a nice transit of Io and its shadow. So if you’ve been thinking about setting up a telescope and showing “eye candy” to the ghosts and goblins, this is a great year for it!

Uranus, too, is well placed for observing. It was at opposition on September 21 – the same day as Jupiter – and is still quite close to its giant neighbor. Look about three degrees up and to the left of Jupiter; there aren’t any bright stars nearby, but you should be able to find it by sweeping with binoculars or a low-power eyepiece. As always, look for a greenish color and that steady, non-twinkling appearance that tells you you’re seeing the disk of a planet, not a point source like a star.

Don’t forget to swing over to Neptune, hanging out off the left tip of Capricornus a bit west of the Jupiter/Uranus pair, pretty much the same place it’s spent most of this summer.

The inner planets – all those lovelies that have graced our early evening skies for the last several months – are disappearing in the twilight. Venus and Mars are together sinking lower in the western sky: start early if you want to catch them. It’s too late for Saturn, which dives past the Sun at the beginning of October and will appear in the morning sky next month. Pluto is also too low to be very rewarding this month. Mercury makes a brief appearance in the morning sky early on, then disappears into the Sun’s glare by mid-month.

So the show this month is all gas giants. If you want to look at one of those rocky, dense inner planets, look at the moon – or get out and enjoy the nice fall weather on our own world!