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Composite video showing what the Earth's sky would look like if Saturn orbited our planet at the same distance as our moon. (YouTube)

What We Need Is A Moon With Rings

by Adam Frank
Jun 24, 2014

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Sure, we have a nice thing going for us here on Earth. The temperature is just right. We have lots of water. The atmosphere is full of tasty oxygen. But even with these appealing characteristics, there is something about our planet that seriously bums me out.

The sky. It's ... well ... dull.

Yes, we have a moon. And, yes, it's a nice moon. There is a "man" or a "rabbit" on it, depending on your state of mind. But, as viewed from the Earth, the moon really is kind of small (just 0.5 degrees out of the 180 degrees of "arc" extending from horizon to horizon).

Wouldn't it be way cooler if there was a much bigger moon in the sky? Or maybe a moon with another moon whipping around it? Or maybe even a moon with rings!

Yeah, a moon with rings — that would be cool.

To get a handle on what our sky would look like if there were something other than our moon in it, I found a nice video that drops the other planets in our solar system into the moon's location.

What this video really gives you is a nice illustration of the relative sizes of the planets. Jupiter, for example, has a radius of about 70,000 kilometers, making it about 40 times larger than our moon (which has a radius of about 1,700 kilometers). So if you put Jupiter where the moon is, it would appear about 40 times larger in the sky.

Now that would make for an interesting sky!

Of course, the unscientific part of all this is that, with anything much bigger than our moon close to the Earth, we'd be living on a very different planet. If Jupiter, for example, was as close as our moon, the tidal forces (the difference in gravitational pull from one side of a body to the other) would be large enough to tear the Earth apart. (HT to my colleague Dan Watson for whipping together a calculation of the Roche Limit for Earth and Jupiter before I could say "boo.")

And speaking of unscientific but cool, here is a video of what the sky would look like if the moon orbited as close to the Earth as the International Space Station. In reality this would be a disaster for both Earth and moon. But it does show what it looks like for an object to "fill" the sky (cover many degrees of arc). The moon's rapid movement in this video is a result of its near-Earth orbit.

So what would be scientifically possible in the awesome-night-sky department? Imagine what it would be like to live on a planet at the edge of a galaxy. Every night a great pinwheel of stars would fill the ENTIRE SKY above your head. Or what if you lived close to a star-forming cloud? Then, every night its gorgeous glowing variegations would reach from horizon to horizon. That would be even better.

So, yes, the Earth is a wonderful place. But its sky could use some help. I'm not asking for much here, folks. Maybe just another small moon? Mars has two moons. C'mon, can't somebody do something about this?


You can keep up with more of what Adam is thinking on Facebook and Twitter: @adamfrank4.

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