SJAA Ephemeris February 2004 | SJAA Home | Contents | Previous | Next

The shallow sky

Saturn in Thin Air and a Double Double

Akkana Peck


“... due to the inclination of Jupiter's orbit multiple satellite events have become much rarer ...”




Keep a telescope handy, keep an eye on the sky for holes in the clouds, and get ready to look at Saturn! It's perfectly placed for observing this month: already up at sunset, it's high in the sky by the time darkness falls, and at its highest, before 11pm, it soars to within fifteen degrees of the zenith, about as high as planets ever get.

Why am I always talking about how high planets are, and observing them when they get high in the sky? Well, generally, the higher a planet appears in the sky, the steadier it will appear. There are two reasons for this. First, you're looking through less air when you're looking straight up than when you're looking horizontally toward the horizon. This isn't immediately obvious: how can there be more air looking in one direction than in another? Here's an easy way to visualize this: take a compact disc, and pick a point on the outside of the center ring. That's you: the clear part in the middle of the CD, below you, is the earth, and the printed or rainbow part of the CD, above you, is the atmosphere. (In reality, the atmosphere would be tiny compared to the size of the earth, but let's not worry about that now.) Next, measure from "you" straight out to the edge of the disc. You don't need a number: if you don't have a ruler handy, just use any straight object you have handy, hold one end at "you" and mark the other end where it intersects the edge of the disc. Now swing your measuring stick by a right angle, so that now it goes from "you" out along a tangent to the "earth". Now the imaginary "you" is looking at the horizon. See how much farther it is to the edge of the "atmosphere"? That's that much more mucky air that you're looking through when you watch something rise or set compared to when it's nearly overhead.

I said there were two reasons. The other is that air close to the ground tends to be turbulent because of all the heat held in the ground even at night. When you look toward the horizon, nearly all the air you're looking through will be showing heat waves - it's as though you were trying to observe over a bonfire.

That doesn't mean you can't look at things near the horizon. I do it all the time! (Though Dave sometimes makes fun of me for my hobby of watching astronomical objects rise and set through trees.) Just be aware that you probably won't see as much detail then as you will when the object is higher in the sky.

Okay, back to the planets. Saturn is a month past opposition, so there won't be as much shadow of the planet on the rings as you'll see in a few months. That means it may look a bit less three-dimensional this month than it will as we pull away from it, even though it's closer now. See if you can see the shadow grow, week to week! The ring tilt this month is 26 degrees, slightly less than last year's maximum; the angle will widen gradually over the next few months and then decrease for the rest of the year. The difference won't be that great, though; we'll still see plenty of ring face for the next several years.

Jupiter rises in the early evening, after it's fully dark, but should be high enough to show plenty of detail before the evening gets too late. It, too, will rise high in the sky by the time it transits (after midnight), though not as high as Saturn.

On February 7, starting at 10:42pm, we'll be treated to a relatively rare event: a double satellite transit, in which Callisto, Ganymede, and Ganymede's shadow all cross the face of Jupiter at the same time. A similar but even better event happens at 9:37 on the night of the 26th, when we'll see Europa, Io, and Io's shadow right behind Io (so we may be able to see a very rare "crescent shadow"); and if this isn't enough, the great red spot will also be visible, south of Io. A few years ago, double satellite or shadow transits were common, and we only got excited about a triple transit. But alas, due to the inclination of Jupiter's orbit (it's farther from the plane of our orbit now than it was a few years ago) multiple satellite events have become much rarer, and we have to take what we can get. So take a look on the night of the 7th, if the weather permits.

Mars, shrunk to a tiny dim ghost of its former self, is nonetheless still visible in the evening sky most of the evening, setting around midnight. It will be difficult to see many features on its six arcsecond disk, though it can be done; more interesting may be to notice its phase. We normally don't think of outer planets as having a phase, but Mars gets noticeably gibbous when it's more than a few months before or after opposition.

Venus, too, is visible in the evening sky, much lower than Mars (and lower than its excellent apparition last month) and setting much earlier. Its phase will be fairly close to half.

Mercury, Uranus and Neptune are all too close to the sun to be observable this month. Pluto has moved into the morning sky, in Serpens Cauda near Xi and Nu Serpentis, and might be findable by a dedicated Plutocrat.

One last comment: the latter half of this month is a good time to look for the "zodiacal light", a faint band of light extending from the horizon upward along the ecliptic after dark. It's fairly faint - usually dimmer than the Milky Way in surface brightness -- and will only be visible from dark sky sites when moonlight isn't interfering. More about the zodiacal light and what causes it, next month.


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