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Mars at Opposition

Week of April 26, 1999

Do me a favor. Go outside and take a look around. I'll wait here.

Back already? Good. I hope you got a good look at the sky. There's quite show going on outside these nights. Right after sunset, and maybe even a little before, it's hard to miss Venus. Shining brighter than any other object in the sky except for the Sun and the Moon, Venus is high in the west around sunset. It's so bright that it's easy to find even before the sky is dark (though it took my neighbor a minute to spot it). Venus is still relatively far away, but it's swinging around the Sun and heading towards us, so it's getting much brighter.

On the other side of the sky just after sunset, a blood red Mars is also pretty obvious. Shining brighter than any nighttime star, Mars is an easy object in the East. On the night of April 24, 1999, Mars reached opposition. That means that the Sun, Earth and Mars were all roughly on the same line. Mars is opposite the Sun in the sky, hence the term. That also means that like the full Moon, Mars rises when the Sun sets, and sets when the Sun rises. It's the perfect time to view it through a telescope, too. It's up all night, and it's as close to the Earth as it can get. Mars' orbit is roughly a circle, about one and a half times the diameter of Earth's orbit. if you draw a picture for yourself, you'll quickly see that the time when Mars is closest to Earth is when they both line up with the Sun. About six months from now, Earth will be on the opposite side of the Sun from Mars, and Mars will be as far away as possible. Since Mars' orbit is actually an ellipse and not a circle, some oppositions are better than others. This one doesn't seem to be a particularly good one: Mars is about 80 million kilometers away, when at closest it can be about 50 million.

At that distance, Mars' disk is 16 arcseconds across, far too small to discern with the naked eye. One arcsecond is about 1/2000 of the width of the Moon as seen from the Earth, so it's a pretty small unit. You'll need a good telescope to see any detail, but if you can find one I highly suggest taking a look (try seeing if there's an astronomy club near you! There will certainly be people observing Mars). Mars is the only other planet in the solar system that has surface features you can see from the Earth easily. Venus is covered in clouds, mercury is too close to the Sun for easy viewing, and the rest are gas giants, and have no real surface! From Earth you can see some dark features (once thought to be patches of vegetation) and even one of the ice caps. They aren't water ice, though; they're actually frozen carbon dioxide. Mars is not a really fun place to be.

But someday it might be. When Mars reaches opposition, it's easier to send probes to it because the distance is relatively small (actually, since both Mars and Earth are constantly moving, you need to launch the probe before opposition, but who's quibbling about details? ;-). It may not be too far in the future when an opposition like this one sees a probe full of not robots and little rovers, but people and supplies. Mars may be a cold barren desert now, but it's not only conceivable but highly probable that someday we will terraform it, or change it to be more Earthlike. What an amazing thing that will be, to see oceans and life abound on a planet where there has been no running water in a billion years. I'd like to see it, and I am surely hoping my grandchildren will.

For more about Mars, Venus, and any other planet you can think of, always head over to Bill Arnett's The Nine Planets.

©2008 Phil Plait. All Rights Reserved.

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