Any eclipse is worth seeing. A total eclipse — where the moon completely blots out the sun, where day turns to night, where solar flares ring the moon's shadow like a crown of flame — that's the eclipse everybody wants to see, the alpha eclipse that eclipses all the other eclipses. Everybody knows this (me included), until I saw this ...
Yes, it looks like a cross-eyed space alien staring out of the darkness, so to make things clearer, let me add one more "eye," like this ...
What are we looking at? On Aug. 20, 2013, NASA's robot Curiosity was sitting on a Martian plain and one of its cameras looked up at the sky and saw the little moon Phobos passing across the face of the sun. Curiosity's camera snapped a picture every three seconds. So what you see here is a sequence. The moon appears on the right side of the sun, moves center, exits left, a passage that took about 37 seconds. Had you been on Mars that day, this (NASA animated its photos) is what you would have seen ...
Obviously, this is not a total eclipse. Phobos, it turns out, is too small to cover the sun. It is, amazingly, only 14 miles wide. Our moon, by comparison, is 2,160 miles across.
So how does this itty bitty moon manage to loom so large against the sun, and how come it's so rock-like, so bumpy around the edges — so utterly gorgeous to watch?
The answer is, Phobos orbits very close to Mars' surface. It's only 3,700 miles up. Our moon, by contrast, is (on average) 239,000 miles away. So, Phobos is sailing very, very near, which is why Curiosity can see it in such detail and why it blots out so much of the sun.
Which Would I Rather See?
If you asked me to choose between a total solar eclipse of our moon, and a chance to catch Phobos voguing in sharp outline while I watch from a Martian plain, I'm going for the Martian option: the Little Guy in Partial Eclipse. Not only is it thrillingly beautiful, it is also, I should mention, a tragedy in motion.
Our moon, the Earth's moon, has been gradually drifting away from us. When the Earth was younger, our moon was 10 times closer than it is now. Phobos, on the other hand, isn't moving out, it's moving in — closer and closer and closer to Mars. What's more, it's slowing down.
These days it circles Mars every eight hours. But in the next 10 to 15 million years or so, Mark Lemmon of Texas A&M University told Space.com, Phobos will slow its speed so significantly that, at some point, it will "get so close that tidal forces from Mars will very likely break it up before it does start grazing the atmosphere and come down."
Oh, No ...
What happens then? When a moon disintegrates, it breaks into hundreds of millions of pieces; those pieces splay, then gather, and (at least for a while) they become a ring — like the rings we see around Saturn. When Phobos goes, "Mars may briefly have a ring system," says Lemmon.
'Goodbye,' The Little Moon Is Saying
Which is why, when you see Phobos in partial eclipse on Mars, you are watching a diva making what will one day be its final appearance in our solar system.
So consider what we've got here: a death spiral, a light show, a dying beauty backlit by the sun, What's more fantastic than that? Yes, total eclipses are still nice, still worth traveling to see, but now that I know what Mars gets to see, I'm switching sides. When it comes to eclipses, Partial is the new Total.
At least when I'm on Mars.
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