Mars: Why do Oppositions Vary?
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Every year we celebrate Earth's opposition to the outer planets Jupiter and Saturn by enjoying particularly good viewing of those planets. And every year
when this happens we see these planets about as good as we did the previous year and as we will the next. Uranus and Neptune are a lot harder to see, but
oppositions are on a similar schedule. Not so with Mars, we have to wait 2 years to get up close and personal. And if that didn't challenge our patience
enough, the wait between really good oppositions is about a decade and a half! Why is that?
We all know that Earth has an elliptical orbit, but really, it's not that elliptical. The closest we get to the Sun is 91 million miles, the farthest about 95. So think of Earth's orbit as a circle stretched by a little under 4%. If you were looking at it from a distance it might be hard to perceive it as other than a circle. But not so with Mars. Its minimum distance from the Sun is 128 million miles, but its maximum is 155 million miles - a difference of 28 million. Compared to Earth's difference of just 4 million that is pretty far. So think of Earth's orbit compared to Mars as placing a circle within an oval, with the circle nudged off center. On one side the circle and oval are fairly close, but pushing it closer on one side makes it all the further on the other. So when oppositions happen on the side where the orbits are closer Mars is far closer than when opposition are on the side where the orbits are very far apart.
In 2003 Earth and Mars were the closest (by just a little!) than they had been in 59, 619 years, about 34 million miles. The next opposition was farther, the one after that farther still. And in 2012 that distance increases to over 62 million miles. That's tipping the scale, in coming years that will improve until in the summer 2018 we'll be at a close point again, under 36 million miles. So when it comes to Mars - take it when you can get it! |
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