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August 13, 2003

There is no specific source for this one, but it's spreading. On August 27, 2003, at 9:51 Universal (Greenwich) Time, Mars will be making its closest approach to Earth for quite some time. In fact, the last time Mars was this close was 59,619 years ago! As I write this, it is already incredibly bright, shining at magnitude -3 or so, making it the third brightest object in the sky (if you don't count Venus, which is too close to the Sun to be seen easily right now).

But how close is close? Well, if you read the news, you might think Mars is about to brush past the Earth! A lot of astronomers and news media are saying about how close it will be and how big it will look, but they are leaving out two crucial facts: Mars will be nearly 56 million kilometers away, and it's only fractionally closer than the last favorable pass.

As Mars goes, 56 million kilometers is pretty good. It can be as far as 400 million kilometers (when it's on the far side of the Sun). Mars is farther from the Sun than the Earth, so we orbit faster. As we catch up to Mars in our orbit, the distance narrows, but 56 million kilometers is about as small as it gets. After that, Mars starts receding again as we pass it.

Mars's orbit is an ellipse, more stretched out than the Earth's. The reason this pass is so good is because Mars happens to be near the closest point in its orbit around the Sun, and we are near our farthest point. That minimizes the distance between us. It's true that this is the closest we've been in nearly 600 centuries, but by how much? Well, in 1971 Mars was about 56.5 million kilometers away. Yes, just 1% farther away! Even with a good telescope and a camera, you'd have a hard time seeing the difference. In fact, the difference is so small it would just barely be detectable using Hubble (for the nitpickers: Mars was just 0.2 arcseconds smaller in 1971, which is close to the resolution limit of Hubble).

Still, the hype about Mars is catching fire. I have received several emails from people who have heard that Mars would be "as big as the full Moon". That's silly! For Mars to get that big, it would have to pass us at a distance of 780,000 kilometers, just about twice the distance to the Moon! I read on one website that through a telescope, Mars would look as if it were as big as the Moon, so it's possible that this is where the silliness started. But on August 26th Mars will only by 25 arcseconds across, 1/72 times the size of the Moon. In fact, that's about half the size needed to be seen as a disk, so to your naked eye it will look like a star. Binoculars will just show it as a tiny disk.

Someone else emailed me concerned that we'd get monster tides from Mars. If Mars were so close that it looked as big as the Moon, we'd get tides about twice what we have now. They'd be big, and there'd be damage, but not catastrophic tidal waves and the like. But it never, ever gets that close. At its current distance, the tides from Mars are teeny tiny. I have a page where you can see how small the tides from Mars are. I even do the math!

Some people just have to make more of things than they are. And reality is cool enough! In fact, this is a great time to go out and see the Red Planet (no, not the movie!). Go find yourself a nearby observatory or astronomy club and check it out. One of my favorite websites, The Universe Today, has a list of global Mars 2003 events. See if there's one near you! There will be other wonders of the sky to see as well. It'll be a night well spent.

For more about this passage of Mars, check out the Sky and Telescope website. They also have information about other times Mars has been close, too. For lots of cool info about Mars, always go to The Nine Planets website.

This page last modified Saturday, 05-Mar-2011 18:03:18 UTC
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