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The Moon Rocks!

Week of March 23, 1998

I have noticed a tendency the past couple of weeks for these Bitesized pages to be a bit of a bigger bite than when I originally started out. Three weeks ago was about the expansion of the Universe, for Pete's sake, and last week was about an asteroid folks thought would impact the Earth!

We need something smaller this week, so let's talk about the Moon for a moment. One of the most obvious things about the Moon is that it always shows the same face to us. This is because it rotates on its axis at the same rate that it revolves around the Earth. If it didn't rotate, we'd eventually see all sides of the Moon.

But there is more to the picture. The Moon's orbit is an ellipse, which means that when it is closer to the Earth it moves a bit faster, and slows down when it is farther away. Since it always spins at the same rate, sometimes the spin is a little faster relative to the Moon's orbital motion than other times. This means the Moon seems to "spin ahead" a bit when it is farther from the Earth, and "spin down" a bit when it is closer. What that means is that sometimes we can get a peek just over what would normally be the edge of the Moon; we get to see a few degrees around the side, as if the Moon were turning its head a bit so we could see the side. The Moon is rocking a little as we see it here from the Earth. This effect is called "libration", and you can see it for yourself.

First, it might help to know when the Moon is closest and farthest from the Earth during its orbit. You can do this very easily by making a short stop at the Lunar Perigee and Apogee Counter courtesy of Fourmilab (perigee and apogee are when the Moon is closest and farthest from the Earth, respectively). Note the dates, and observe the Moon at perigee. One easy way to see libration is to note the position of one of the Moon's big blue spots (called "mare" (mar-ay), which is Latin for "sea") relative to the the edge of the Moon's disk. Pick one as close as you can to the edge. Then go out and reobserve the Moon at apogee; has the mare moved a bit? Is it closer or farther from the disk?

Ancient astronomers knew about libration, and were able to observe it with nothing more than the naked eye and a good memory. Can you do as well?

For more info on the Moon's orbit and libration, I highly recommend the web page by John Walker at Fourmilab. This has some math, but not much, and has a nice description of how the Moon behaves in the sky and as it orbits the Earth.

For some info about the effect on the Earth of the Moon's changing distance, check out my page about the effect of the Moon's gravity which compares it to the rest of the planets.

©2008 Phil Plait. All Rights Reserved.

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