That’s no moon– wait, yes it is

Bad Astronomy
By Phil Plait
Dec 1, 2005 9:00 AMNov 5, 2019 6:48 AM


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The Cassini Saturn probe never seems to disappoint. This latest image is another stunner:

That image shows Saturn's weird moon Mimas, which is uncannily like the Death Star, floating in front of Saturn's magnificent rings. Cassini was pretty far out when it took this image; nearly twice as far from Mimas as the Earth is from the Moon. I've written about Mimas before, but while poking around the web to find out more info just now, I found out that Mimas is not even close to being a sphere! Check this out:

That's amazing! I had no clue it was so ovoid. This image exaggerates the effect somewhat, the top part of the moon as seen here is in shadow, so it makes Mimas look more egg-shaped than it really is. But the equatorial diameter is 10% larger than the polar diameter, which is a lot, given its 400 kilometer average diameter. Compare that to the Earth, where the equatorial diameter is only about 0.6% bigger than the polar diameter. The Earth is far more round than Mimas. Earth's oblateness is caused by its rotation: basically, it's just the centrifugal force^* making the Earth bulge. Mimas doesn't spin nearly fast enough for that to be the reason. So I'm not sure why Mimas is so goofy looking (tides from Saturn? Maybe; it's closer to Saturn than the Moon is to Earth). I'll ask Carolyn Porco, leader of the Cassini imaging team, when I meet her at the Amaz!ng Meeting in January. Gloat gloat. * Yes, the centrifugal force is real. Please don't leave angry comments or emails here. The centripetal and centrifugal forces are the same thing, just seen in different frames of reference. People get all snarky about this, and I don't know why. If you sit in a car that is making a turn, you darn well feel a force pushing you. If there were no force, you wouldn't feel it. It's really that simple.

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