What We Have Been Missing

What We Have Been Missing

Adam Frank for McGraw-Hill

Did you see it? Did you notice the change? If you are like most of us, you probably were not aware there was any difference in the sky. If you are like most of us, you probably don't have much reason to look up anymore. But if you did stop and look up, if you did take a few moments during the last few weeks in June to scan the night sky after sunset, you would have seen it. It was here above the horizon - the red planet hanging in the sky bigger and brighter than usual. This was Mars in opposition…a mini solar system event which only comes around every couple years or so. While you probably missed it, your great, great, great grandfather never would have and in that fact there lies quite a story.

In astronomy the term opposition means a planet appears directly opposite the Sun in the sky. When the Sun goes down in the West, the planet comes up in the East. Opposition only occurs for planets that are farther from the sun than the Earth (can you draw a picture and convince yourself why?). Physically, opposition means the planet (Mars in this case), Earth and the Sun are all, more or less, lined up. If you think about it for a moment (or go back to your picture) you will see that when a planet is in opposition, it is just about as close to the Earth as it is going to get for that (Earth) year. That means it should be as big and bright in the Earth's night sky as it is going to get as well. In the days before television and streetlights and all-night Burger-Gunk restaurants, people noticed the night sky. It was there as the great mystery, facing humans down every night. They also noticed if something happened such as a planet getting brighter over a period of weeks and then fading back down. So there was a time when oppositions mattered. They were real events, the cosmic equivalent of a WWF smack down coming to town.

For Mars, the closest distance possible to Earth is about 55 million kilometers. During the opposition that happened in June 2001, Mars did not get that close. It was "only" 67 million kilometers away (about 42 million miles). So, during the opposition Mars was particularly easy to spot. How can you find a planet like Mars? Planets are always easy to differentiate from stars. They don't twinkle. That is because they're close enough to actually appear as little disks. Stars never appear as more than pinpricks of light. That difference means the image of a planet does not get bounced around by atmospheric turbulence the way a point-like star does. Once you get that twinkle distinction down you should be able to recognize Mars just by looking for a bright reddish object.

Putting it all together, if you were to go out and look on a regular basis, if you actually followed the goings on in the night sky the way your ancestors did, you would have seen Mars REALLY looking different over the last month or so. You would have looked up and been hit in the head by how noticeably brighter it looked. That is what the opposition is all about.

In terms of getting a good view of the red planet the best oppositions come when Earth crosses between Mars and the Sun about the same time Mars reaches its closest approach to the Sun. When Mars is as near to the sun as it can be and we are in between, a little reasoning shows Mars must be as near to us as it can be. This year Mars gets closest to the Sun in October so June's opposition was pretty decent as these things go. As the planets whirl around the Sun guided by the laws of physics there's a regular cycle for the really good Martian oppositions. The closest ones come about every 17 years. The last one was in 1988 - and the next one will be in the year 2003, when it comes 34 million miles from Earth; 8 million miles closer than the June 2001 opposition.

If you were a farmer or sailor a few hundred or thousand years ago these cycles on the night's canvas would have been more than abstractions. It is something we, in the modern world, have lost sight of. It's a pity because on a deep level the sky is far more interesting than some of the other things we waste time staring at.

Check Out These Websites

Mars at Opposition
http://marswatch.tn.cornell.edu/hst99.html

Mars Opposition 1
http://www.earthsky.com/2001/es010612.html