Observing the clouds of Jupiter

Jupiter is at opposition this month! This means that it's your best chance (until next year) to observe all the fascinating detail of Jupiter's turbulent atmosphere.

The first time you look at Jupiter, you probably saw the Galilean moons right away. Those are the four large moons that Galileo discovered: Io, Europa, Ganymede and Callisto. On nearly any clear night you'll see some or all of these moons.

Look a little longer, and in almost any telescope you'll notice horizontal stripes across the planet. These are the north and south equatorial belts, pale red bands just north and south of Jupiter's equator. We abbreviate them as NEB and SEB. They're formed by clouds of different temperatures, heights, and compositions within Jupiter's atmosphere, in contrast with the off-white "zones" between them. Think of them like the Jet Stream — a band of cloud that stays in roughly the same place, while still changing shape and wandering slightly north or south.

Almost as easy to see are the polar regions (NPR and SPR for north and south), greyish or blueish patches over the poles. Again, we're seeing clouds, not actual polar ice like we might see on Mars or our own planet.

How do you tell which direction is north and which is south? After all, when you're looking in a telescope, the planet is probably rotated, maybe flipped, maybe even mirror-reversed if you're looking through a refractor or SCT. Well, the easiest way is to keep your eye glued to the eyepiece while you bump the scope very slightly in the direction of north — put your hand on the south side of the tube, and just bump the scope a little. That will make the planet in the eyepiece appear to move south. Voila, you know which polar region is which!

Okay, so you've looked at Jupiter a few times, and you've seen the red stripes and grey polar regions. Don't stop there! There's a lot more to see, but it takes some concentration and practice to see it. Planetary observing is a skill just like deep-sky observing, and the more you concentrate and practice tonight, the more you'll see tomorrow night. Pull up a chair and sit down! Get comfortable! Breathe deeply! Those all help you see more, too. And of course, just like I always do, I'll recommend sketching as a way of focusing your attention and helping you see more.

"What about the Great Red Spot?" I hear you asking. Of course we want to see the GRS! But don't believe everything you read: first of all, it's not very red, and hasn't been in years. If you want to see the Great Pale Pink Spot, though, you've come to the right place. It's in the SEB, so look first for a swelling in the SEB, a place where the band splits and widens. That's the Red Spot Hollow, the place where the GRS lives, and sometimes it's easier to see the Hollow (since the SEB is easy to see, but the GRS isn't) than to see the spot inside it.

In excellent seeing, sometimes you can see turbulent swirls inside the GRS, and in the split between the two halves of the SEB following behind the SEB. Seeing that steady isn't too common around here in winter, especially in an El Nino year, so don't be too upset if you don't see details like this right away — but keep looking, and every now and then you'll hit a clear, steady night that will amaze you.

Get that clear, steady night and a good scope, though, and the features I've mentioned so far are only the beginning. Take a look at the sketch, made by Jane Houston through a 7" refractor on a night of excellent seeing at Fremont Peak. In addition to the equatorial bands, there are many other bands called temperate bands which should be visible on a decent night. In addition, there's the elusive equatorial band (EB), very narrow and much paler in color than the other bands. Some years it's not visible at all; other years it's fairly easy.

The equatorial bands sometimes get long streaming fingers trailing off them into the nearby lighter zone, usually toward the equator. These are called "festoons", and they're lovely to watch. They change fairly rapidly; you can often see changes in a few days or a week, while most features on Jupiter persist longer).

Remember that turbulence between the two segments of the SEB? Sometimes a particularly turbulent area will show up as a white spot. White spots run in packs — if you see one, there are probably several others nearby, as Jane's sketch shows. Smaller, dark spots (they may appear blue, or red, or just "dark") are called "barges", and appear to be cooler areas which, like festoons, come and go fairly quickly.

Sit back and think about the scale of what we're seeing. These white spots are nearly as big as the earth; the GRS is much larger. Planet-sized swirls of turbulence, appearing and disappearing in a few weeks, and you can watch them change before your very eyes!

If that gets to be too much, take a break and go look at Saturn. It's nicely placed for observing all month, leading Jupiter in the sky by a few hours and showing a ring inclination of almost 27 degrees. Lovely! When it passed the Crab nebula last month, most people couldn't see both at the same time, but it sounds like everybody was happy to have a good excuse to go stare at Saturn for a while.

Mars rises about 3 a.m., but it never gets very high this month before it's overwhelmed by the light of dawn. We'll have to wait a few months more before we can see much on the small red planet. Mercury and Venus, too, are visible low in the morning sky.