Cosmology, Biology, Geology, Biogenesis
"This ain't no mudd club, or CBGB. I ain't got time for that now."


Terra and Luna with sizes and distance to scale
Line below Terra is distance from Terran center to barycenter . The line below the moon shows the perigee and apogee distance differential.
Click for a larger version.

One object does not orbit another object. Both objects orbit a point that is exactly centered regarding the distribution of their mass. With bodies of sufficient mass differential, the center of mass may be immeasurably close to the larger body's own center of mass.

This means that unless both bodies are perfectly spherical, then rotation will change mass distribution to some degree. In the case of Terra and Luna, the lunar tides create bulges on Terra, which is already shaped as an oblate spheroid. However, Luna's orbital period is slower than the earth's rotation. This means that Terra is spinning while Luna tries to "grab" the ocean and hang on to it. The net effect is that Terra moves the bulges slightly in the direction of its rotation. This pulls the tidal bulges slightly out in front of Luna, which changes the mass distribution.

With the mass distribution slightly skewed to one "side" (as viewed from above the ecliptic ), this means that there is a bit of a tug to speed up Luna's orbital period.

For a strange analogy, consider you are ice skating (in a lovely silver faux-cratered leotard) in a circle around a giant disco ball (done up in blue, green, and white) that's within your reach. The disco ball is six times your weight (and mass) and spins almost thirty times faster than you go around it. If you were to reach out and try to slow it down by just holding on to it, the friction of your touch will slow it down slightly. However, the disco ball is now slightly pulling on your arm, which is speeding you up. As is commonly known, when you spin something, it attempts to move farther away from the center of spin. Since you're not tethered to the disco ball (as Luna is not tethered to Terra), you will actually get slightly farther away, until you can no longer reach the disco ball.

So Luna's getting a little tug to speed it up, which sends it sailing away from Terra at the rate of a few centimeters each year (about the rate of fingernail growth). Of course, it's a two-way street, so Terra is being tugged backward, which means it spins slower each year (at a much slower rate, since Luna is less massive). This will continue until one of two things happen - either Luna "skates out of reach" or Terra slows its spin to match Luna's "skating." Since the reach of gravity dimishes far less quickly relative to any person's physical reach, the second scenario is the inevitable outcome. However, such a thing will take about another two billion years.

Eventually, Luna will slow Terra to a rate that matches Luna's orbital period. The tides will now stay put, the distance between the two will no longer change. Terran days will be 47 of our current days; the same duration as the lunar cycle. Even before then, only one half of Terra will ever be able to even see Luna, since rotation and revolution will be nearly synched.