Why The Moon Is Receding

As the Earth's speed of rotation diminishes owing to tidal friction, its angular momentum falls. The angular momentum of a body is a measure of its disinclination to stop rotating (or indeed to speed up), whether that rotation is in the form of spinning on its axis, or revolving around another body. An example of the latter is any planet orbiting the Sun, or the Moon orbiting the Earth. A body's angular momentum depends upon its total mass, the distribution of that mass, and its rotation rate. The total angular momentum of any system is a quantity that is absolutely conserved (remains the same). An example is a pirouetting ice-skater. With arms outstretched her spin rate may be slow, but as she draws her arms down to her sides, the rate of spin increases. Because the mass distribution has been changed, the spin rate alters to compensate and thus keep the angular momentum constant.

In the case of the Earth there is braking due to tidal drag, because the continents prohibit the free movement of the tidal swell right around the globe, and in consequence the planet's spin angular momentum reduces. We have just seen, though, that the angular momentum must remain the same. Evidently, something else has to be happening here if the laws of physics are to be obeyed.

How is this achieved? I noted above that it is the total angular momentum of the system that is conserved. Here we are considering the Earth—Moon system as a whole. As the spin angular momentum of the former drops, the angular momentum associated with the orbit of the latter must increase. To make this happen, the Moon recedes from our planet, very slowly. And that is why the lunar laser ranging experiments indicate that our natural satellite is receding from us at about an inch and a half every year.

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