View Full Version : Moon's Wobble
07-27-2006, 09:39 PM
I was wondering if the wobble of the moon could be due to the effect of the gravity of the sun as the moon travels around the earth. I don't have the equipment or expertise to verify this, but if anyone does, this would be very interesting to explore. The heavier side of the moon would be drawn toward the sun just as it is toward the earth.
07-27-2006, 09:56 PM
You appear to be referring to Chronos' report, Why does the same side of the moon always face the earth? (http://www.straightdope.com/mailbag/mlunarrotate.html), which talks about the moon's wobble. Please, in the future provide a link to the column to make it easier for the rest of us to know where to look for the answer.
Problem is, only the questioner talked about a "heavy side" to the moon, and that questioner understood about as much astronomical detail as the average Pokemon monster. There is no heavy side to the moon, the earth does not draw the moon by its non-existent heavy side, and that means that the sun doesn't either.
You may be referring to the slight asymmetry of the tidal force as creating a heavier side, but that's not an accurate way of looking at it. And the earth's influence on the moon is much stronger than the sun's. The sun's gravity may make a need for a minor correction factor in the wobble, but it is not the cause of it.
John W. Kennedy
07-28-2006, 08:41 AM
Considering that the Earth's nutation (secondary wobble) has a period of 19 years, which means the sun's gravity is obviously involved, I dare say the moon has a similar deal.
07-28-2006, 11:42 AM
The primary cause of the wobble is the fact that the rotation is at a constant speed, while the orbital motion is not. This would occur even if the Moon and the Earth were the only two objects in the universe. There are, however, a number of other effects which cause smaller wobbles, and some of them do involve the Sun. The tidal effects of the Sun on the Moon would be insignificant compared to the Earth's (less than one percent as large), though.
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