Top Document: [sci.astro] Solar System (Astronomy Frequently Asked Questions) (5/9) Previous Document: E.14.1 Why doesn't the Moon rotate? Next Document: E.14.3 Is the Moon moving away from the Earth? (and why is Phobos moving closer to Mars?) See reader questions & answers on this topic! - Help others by sharing your knowledge Author: Laz Marhenke <laz@leland.Stanford.EDU> When it first formed, the Moon probably did not always show the same face to the Earth. However, the Earth's gravity distorts the Moon, producing tides in it just as the Moon produces tides in the Earth. As the Moon rotated, the slight elongation of its tidal bulge was dragged a bit in the direction of its rotation, providing the Earth with a "handle" to slow down the Moon's rotation. More specifically, the tidal bulge near the Earth is attracted to the Earth more strongly than the bulge away from the Earth. Unless the bulge points toward the Earth, a torque is produced on the Moon. If we imagine looking down on the Earth-Moon system from the north pole, here's what we'd see with the Moon rotating at the same rate as it goes around the Earth: Earth Moon __ / \ ____ ^ | | / \ | \__/ \____/ Orbiting this way Tidal bulge *greatly* exaggerated. What if the Moon were rotating faster? Then the picture would look like: Earth Moon __ / \ ___ ^ | | / ) | \__/ (___/ Orbiting this way Rotating counterclockwise; Tidal bulge *greatly* exaggerated. If it isn't clear why the tidal bulge should move the way the picture shows, think about it this way: Take the Moon in the top picture, with its tidal bulges lined up with the Earth. Now, grab it and rotate it counterclockwise 90 degrees. Its tidal bulge is now lined up the "wrong" way. The Moon will eventually return to a shape with tidal bulges lined up with the Earth, but it won't happen instantly; it will take some time. If, instead of rotating the Moon 90 degrees, you did something less drastic, like rotating it one degree, the tidal bulge would still be slightly misaligned, and it would still take some time to return to its proper place. If the Moon is rotating faster than once per orbit, it's like a constant series of such little adjustments. The tidal bulge is perpetually trying to regain its correct position, but the Moon keeps rotating and pushing it a bit out of the way. Returning to the second picture above, the Earth's gravitational forces on the Moon look like this: ___ F1 <-----/ ) F2 <-------(___/ F2 is larger than F1, because that part of the Moon (the "bottom" half in the drawing, or the half that's "rearward" in the orbit) is a bit closer to the Earth. As a result, the two forces together tend to twist the Moon clockwise, slowing its spin. Over time, the result is that the Moon ends up with one face always facing, or "locked," to the Earth. If you drew this picture for the first case, (where the Moon rotates at the same rate that it orbits, and the tidal bulges are in line with the Earth), the forces would be acting along the same line, and wouldn't produce any twist. Another way to explain this is to say that the Moon's energy of rotation is dissipated by internal friction as the Moon spins and its tidal bulge doesn't, but I think the detailed force analysis above makes things a little clearer. This same effect occurs elsewhere in the solar system as well. The vast majority of satellites whose rotation rates have been measured are tidally locked (the jargon for having the same rotation and orbital periods). The few exceptions are satellites whose orbits are very distant from their primaries, so that the tidal forces on them are very small. (There could be, in principle, other exceptions among some of the close-in satellites whose rotation rates haven't been measured, but this is unlikely as tidal forces grow stronger the closer to the planet the satellite is.) Pluto's satellite Charon is so massive (compared to Pluto) that it has locked Pluto, as well as Pluto locking Charon. This will happen to the Earth eventually too, assuming we survive the late stages of the Sun's evolution intact. :') User Contributions:Top Document: [sci.astro] Solar System (Astronomy Frequently Asked Questions) (5/9) Previous Document: E.14.1 Why doesn't the Moon rotate? Next Document: E.14.3 Is the Moon moving away from the Earth? (and why is Phobos moving closer to Mars?) Part0 - Part1 - Part2 - Part3 - Part4 - Part5 - Part6 - Part7 - Part8 - Single Page [ Usenet FAQs | Web FAQs | Documents | RFC Index ] Send corrections/additions to the FAQ Maintainer: jlazio@patriot.net
Last Update March 27 2014 @ 02:11 PM
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