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The Shallow Sky

It's Time to Look at Mars!

Akkana Peck


Get out your best telescope and your shortest eyepiece - it's time to look at Mars!

This month's Mars opposition shows us the red planet in its closest approach since 1988. Opposition - the point at which Mars is opposite the sun in our skies - occurs on June 13th, but due to the planet's eccentric orbit, closest approach to us (42 million miles or .45 AU) will occur a week later, on the 21st, at which point its normally tiny disk will have grown to a size of 20.8". And although the few weeks before and after closest approach are the best time to see detail on the planet, it won't shrink much for a few months after opposition, so we'll be able to get fairly good looks from now until early August.

The catch is that Mars' orbit takes it very far south this year; here in San Jose, it will only rise to less than thirty degrees above the horizon. That's low enough that getting steady seeing will be a real problem for Northern California Mars observers. If you're planning a trip south, consider taking your telescope along. If you're not, try to find high ground; observing from a mountaintop can at least take you up out of the city smog and get you into slightly thinner air, which might help with steadiness.

What can you see on Mars? In a telescope as small as 80mm, you should be able to see at least one polar cap - probably the southern polar cap, since Mars will be very nearly at one of its equinoxes, and it's the southern hemisphere which is coming out of winter. And you can see dark and light features on the planet. Some of the most obvious features in any small telescope are Syrtis Major, a large dark triangular region, and nearby Hellas, a light-colored impact feature. These two should be well placed for evening observing in late May and early June and again in the first two weeks of July. A bit farther east, the complex of Sinus Sabaeus and Sinus Meridiani (including the so-called "eye of Mars", an oblong feature with a dark spot in the middle which looks like a human eye) are visible when the seeing is good. Farther southeast are Mare Acidalium and Niliacus Lacus - prominent dark features with the mellifluous names so characteristic of Mars. Still farther east is the Tharsis plateau, home of the huge volcano Olympus Mons (also nearby is Valles Marineris, which dwarfs our own puny Grand Canyon), but details on Tharsis are hard to see; some observers have been able to see clouds above the Tharsis volcanos. Lowell probably saw Valles Marineris himself (and thought it to be a canal built by a dying civilization), and a few other observers have been able to see it visually as well.

Mars' day is only 40 minutes longer than our own, so on two successive nights you will see close to the same features at the same time; over a week or two, the planet will rotate to give you a look at the features on the other side. And Mars' weather does change, like our own; if you watch the red planet regularly over the course of the opposition, you'll probably see times when detail is obscured (due to dust storms), areas that appear light when no light features should be there (clouds), and the shrinking of the south polar cap and, perhaps, the growing of the northern one. For some people, using colored filters can help bring out some of these details; other observers don't find filters very helpful, since they can introduce glare and cut light transmission.

How do you figure out what you're looking at on Mars? Once you get oriented, if you observe it every few days, you'll recognize features as they move; but how do you get oriented the first time, or after you've gone a week or two without looking? The easiest way is to use a program which shows you an image of Mars at a particular time. If you don't already have a planetarium program that shows Mars, check my Mars Observing FAQ at http://www.shallowsky.com for links to where to download Mars software. (I'm working on a program of my own; stay tuned and maybe I'll get it ready in time.)

One more challenging target: the Martian moons. They're both relatively faint (10.9 for Phobos and 12.0 for Deimos), but what's worse, they're very close to their very bright parent planet. Phobos, the brighter moon, is actually more difficult to see, because it's so close to Mars (less than one Mars diameter at most) that it gets lost in the glare. To see them, get Mars out of the field (preferably with a narrow-field high power eyepiece which doesn't distort the image much at the edge of the field), or make an occulting bar which sits inside your eyepiece near the field stop and blocks the light from part of the image. Deimos has been glimpsed with apertures as small as 6", and Phobos with telescopes down to 8".

Other planets: Pluto is also at opposition this month, on the 4th (with closest approach to us a day earlier - Pluto, like Mars, has an eccentric orbit). It's in Ophiuchus, at magnitude 13.8 and a distance of 29.4 AU. Jupiter and Saturn, alas, are hidden behind the sun, and so is Mercury. Uranus and Neptune are in Capricornus, rising around midnight. Venus is in the morning sky, reaching greatest western elongation on the 8th.


Mail to: Akkana Peck
Copyright © 2001 San Jose Astronomical Association
Last updated: July 19, 2007

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