28 October 2010
A friend of mine sent me this Ray Bradbury quote:
A few years back, one dreadful boy ran up to me and said, “Mr. Bradbury?”
“Yes?” I said.
“That book of yours, The Martian Chronicles?” he said.
“Yes,” I said.
“On page 92 where you have the moons of Mars rising in the east?”
“Yeah,” I said.
“Nah,” he said.
So I hit him. I’ll be damned if I’ll be bullied by bright children.
Now, I don’t condone hitting bright children, but this made me laugh, because the kid is right. Okay, half right: Deimos does rise in the east, but it is really sluggish. Phobos on the other hand has a true retrograde (a.k.a. backwards) orbit: it orbits faster than the planet spins, so it rises in the west and sets in the east. In fact, because it orbits opposite the direction that the planet spins, the tides that it raises in Mars are gradually stealing its orbital energy, causing it to spiral closer and closer to the planet. In about 8 million years, Phobos will be close enough to Mars that gravity will tear it apart, forming a lovely ring: