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Question:
What would a "day" look like on Mercury? on Uranus? With respect to the apparent path of the Sun to an observer on the surface of these planets...I had read an article that the Sun would appear to make a retrograde movement on Mercury...is that true? Why?/Why not? ...and which orientation does Uranus take as it rotates on its side around the Sun (does it keep one pole always facing away?)



Replies:
An interesting question. I will try to sketch out an answer, though I am not entirely sure. First imagine that the Earth rotated on it is axis at the same rate it does, but in the opposite direction. Then a "day" would be the same length, but the Sun would "rise" in the west and "set" in the east rather than rising in the east and setting in the west as we observe. This is how such things could be reversed.

Now, suppose that we could slow down the rotation so that the Earth rotated on its axis in one year instead of one day. Then the rotation would cancel the motion of the Earth around the Sun, and the Sun would never rise or set on a given spot. One side would always be "day" and the other side always "night". This is the situation with the Moon, which always keeps the same face towards the Earth. That is why the Moon always looks the same to us. Now let us consider Uranus. It rotates on it is side, but this does not really affect the way a day looks as long as Uranus rotates in much less time than it takes for it to go around the Sun. What will get really wacky are the seasons. A minor point here is that the "surface" of Uranus we see is really the cloud tops, which do indeed rotate in much less than a "year" on Uranus. But there is nothing to stand on to watch the sunset there, and no one really knows where the "surface" is if there is one.

Let me back up a bit. A day is the time the planet takes to spin on it is axis, like a top spinning. A year is the time for a planet to go around the Sun. (You probably already knew this, but just in case.)

Now let us try Mercury. This planet is in a resonance where it rotates in 2/3 the time it takes to go around the Sun. Since it "spins" more slowly than it goes around the Sun, the Sun will indeed "rise" in the west instead of the east. (Think about the case where the Sun "stands" still. If rotation is faster, then the Sun must move "forward" i.e. the way it does on Earth. If rotation is slower, the Sun must move backwards.)

Daniel N Koury Jr


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