Mercury's Rotation

Its like the moon, which always shows the same side to the earth. They thought that since its so close to the sun, it would get the same effect.

Basically, think of the moon as being kind of egg shaped. (dramatized) The fat end faces the earth. Why? because the gravity of the earth distorts the spherical shape of the moon. the earth draws the moon's matter towards itself, causing it to bulge. At the same time, it slows the moon's rotation, causing it to become synchronized with its orbit of the earth. Thus the same, fat side always faces the earth.

Incidently, the moon is doing the same thing to the earth, albeit much more slowly. The earth day is gradually slowing, and perhaps one day the earth's rotation will become synchronized with the moon's orbit, causing the moon to appear stationary in the sky. Scientists believe this is the case with the ex-planet Pluto, and its relatively huge moon Charon.

Technically, "doesn't rotate" isn't accurate. What they used to think was that the period of rotation (how long it took to go around once on its axis) was exactly the same as its period of revolution (how long it took to go once around the sun). This is also the situation the Moon has with respect to the Earth.

If that's still confusing, try modeling it. Set a chair (or ask someone to stand) in the middle of a room where you have enough space to walk all the way around. Start out facing the chair and side-step around in a circle so that you're always facing it. Once you get back to your starting point, you've made one revolution around the chair. Notice that along the way, even though you were always facing the chair, you also faced all four sides of the room at different times. That means you turned around once (made one rotation around your axis).

The moon's rotation is one revolution per orbit. In other words, a point on the equator rotates around the moon's axis so that it always faces Earth (just as described above).

If the moon did not rotate about its own axis, it would appear to rotate as observed from Earth. We would see, at different times of the month, different "sides" of the moon.

And yeah, Maryn, about that story. Several of them, actually. One I remember in particular is written by Larry Niven (his first published story, December, 1964): The Coldest Place. The story was rendered obsolete before it was published, due to advanced probes of the planet Mercury.

Maryn said:

Any chance I could get a nice green giraffe to stand in the middle of the room? That would make it more memorable...

Maryn, just poking fun

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That could work

Maryn said:

I love it when a question well beyond me comes up and I know someone at AW can probably answer it.

When I was a kid, I learned in school that the planet Mercury, alone of the nine, did not rotate. (Man, I'm old, huh?) Now we know that of the eight, none fails to rotate.

My question is, for Mercury alone to have been thought to be non-rotating, with one side always facing the sun, the other always facing away, there must have been a reason, an observation or a measurement or something which led to that conclusion, now disproved.

So my question: Why did astronomers used to believe that only Mercury did not rotate?

Maryn, remembering a particularly good science fiction story which no longer works if Mercury rotates

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There ought to be some clear explanation of why they thought that in some older astronomy book. And of course this (that they thought Mercury had one side always facing the Sun) is called being tidally locked, just as our Moon is tidally locked to the Earth, always having only one hemisphere visible from Earth.

I recall that Mercury rotates at some "harmonically locked" rate, ...

heck, I just found everything you need to know, and more, here:
http://en.wikipedia.org/wiki/Mercury_(planet)
The third sentence says:

It completes three rotations about the axis for every two orbits.

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which may odd at first, but as I said above, I've heard this described as being "harmonically locked." It's a little like an egg or a football, and when it's nearest the Sun (perihelion), its longest axis is pointing toward the Sun (when tidal forces are greatest and most strongly makes the long axis point toward the Sun), and the "end" that points toward the Sun every perihelion alternates with every orbit, thus it rotates 1 1/2 times for every orbit.

And here's the explanation of why astronomers thought for so long that Mercury was tidally locked (Mercury's actual rotation rate is part of the explanation):
http://en.wikipedia.org/wiki/Mercury_(planet)#Spin.E2.80.93orbit_resonance
The short-short is that every picture of Mercury showed the same side facing the Sun because every time the Earth-Mercury-Sun system makes it most convenient to take good photographs to see Mercury's surface features, that's where Mercury is in its rotation!

I would just like to say, in my heart, it's still:

My very educated mother just served us nine pizzas.

Nine-4-Eva