Mars Observing 101

The big news this month is the Mars opposition.

What is an opposition, anyway? The term means that the planet in question is on the opposite side of the earth from the sun. In other words, it's highest at midnight. (At least, more or less midnight - you do have to allow for details like daylight savings time and the equation of time.) More important, an opposition is when a planet is closest to us, because we and the planet are both on the same side of the sun. With an outer planet like Jupiter which is always far away, that doesn't matter much; but for a planet that's small and close, like Mars, it makes a huge difference in how big the planet appears.

Okay. To be completely truthful, the opposition isn't this month. That won't happen until November 9th. But because of eccentricities in Mars' orbit and our own, the closest approach between Mars and Earth happens at the end of this month, on October 29. Just in time to set up a telescope for Halloween and show the kids a Halloween-colored planet!

By then, Mars will be rising about half an hour after sunset. Its size will be 20.2", big enough to show a nice disk with surface features in a telescope of any size. (Remember I said opposition makes a big difference in a planet's apparent size? Mars began 2005 at an apparent size of 4.5".)

Even 20.2" might seem disappointing to observers who remember the 25" disk Mars showed for the 2003 opposition (AS BIG AS THE MOON! Well, give or take a factor of 70 or so). But don't be disappointed! In 2003, Mars only rose to 37 degrees above the horizon at its best, way down in the murky, unsteady haze. This year, it'll be 60 degrees up, in the clear, steady air that planetary observers love best. You'll be able to use much higher magnifications (assuming we get some decent weather) and you may be able to see much more.

What can you see? The map here, adapted from Daniel M Troiani's ALPO Mars map, shows most of the features you'll be able to see in a typical amateur telescope on a good night. South is up.

The first time you point a telescope at Mars, you won't see anything like this. Don't panic! Observing Mars takes practice. It's one of the most difficult planets to observe, for several reasons: it's quite small even at opposition; it's only that large for a few months every two years, so we don't get much practice at it; all its features are subtle shades of brown and orange, no sharp features. At first you may only see one or two light blobs and one or two dark blobs, but keep at it and you'll gradually see more.

(I have to put in a plug for sketching: it's a great way to train your eye and think about what you're really seeing. Don't worry if your Mars sketches look awful. Mine do too. But whether or not you're making good art, the act of sketching what you see will help you see more. I promise!)

There's one more reason Mars is a harder target than other planets: it rotates fast (its day is just a hair longer than our own), and its axis is inclined slightly more than our own. The result? It's hard to tell which side we're looking at, or even where the poles are so you can tell north from south.

The best way to figure out what you're looking at on Mars is to use a computer program: either a good planetarium program, a special purpose program, or a PDA program, to tell you how it's oriented right now. I have a list of programs I know about on my Mars page, http://shallowsky.com/mars.html - if you know of one I've missed, please let me know.

In addition, a Mars globe is really helpful. The only real globe you can buy is expensive and too large to carry on star parties; but there are several globes and icosohedrons available on the web which you can print out and use to make your own Mars globe. There's also an inflatable Mars beach ball available which has a surprisingly good map. Unless you're adept at mentally warping rectangular maps onto circles, a globe will help you a lot with identifying features.

So you've got your globe, your software and a map and you're looking at Mars. You see a dark smudge and a light smudge. What are they?

Mars' southern hemisphere will be tilted toward us throughout this pass. So if you see any polar cap, it'll be the south pole. However, it's late summer in Mars' southern hemisphere, so the polar cap will be very small and hard to see. If you see something big that looks like a polar cap, it's probably Hellas, a huge impact basin which sometimes fills with fog and looks more icy than the icy polar caps themselves.

The most obvious dark spots you'll see are Syrtis Major, the Sirenum/Cimmerium complex, and the Erythraeum area. These are all distinctive in shape - Syrtis Major looks like India or Africa with bright Hellas on one edge, Sirenum/Cimmerium are two elongated areas, and Erythraeum is a larger and more general dark area.

Once you orient yourself and figure out which part of the planet you're looking at, you can start to look for finer features. See if you can see Sinus Sabaeus and Sinus Meridiani. Look for the "Eye of Mars" near Solis Lacus, an area which looks surprisingly like a human eye. You'll probably be able to see Niliacus Lacus (isn't Mars nomenclature wonderful?) and Acidalium to the north of Erythraeum - they're quite dark and usually easy to see.

Look for lighter areas around the Tharsis plateau, where most of Mars' big volcanos are. The volcanos are big enough that they generate "orographic clouds", air driven upslope so that it cools and the moisture in it condenses, just like the clouds you sometimes see over Mt. Hamilton when the rest of the sky is clear. Weather features come and go, so keep watching and see whether you see changes in the lightness of the Tharsis area. Blue or green filters may help a little.

Speaking of weather changes, watch for dust storms. Sometimes you look at Mars and don't see the features you expect to see. On a desert planet like Mars, you never know when a dust storm might blow in and cover everything. Usually they only last a few days, but sometimes they last for weeks. If it happens, try not to be too disappointed at not being able to see features - think of how mind-boggling it is to be able to watch the weather on another planet. Maybe that's worth missing a few features.