American Scientist

To the Editors:

Regarding P. Surdas Mohit's article "The Two-Faced Moon" (May-June), to what extent are the two faces of the Moon distinct due to the effect of orbital forces—centrifugal and centripetal—on the Moon's mass distribution of the various mineral aggregates and processes described? Given that the Moon rotates on its axis only once a month, are not such purely motion-related factors more significant than the various elaborate and speculative geothermal and geochemical factors in terms of explaining why certain rock types of various densities have "surfaced" with more frequency on one side of the moon than the other?

Alan Hull
Conyers, GA

Dr. Mohit responds:

This is an important question, as is the converse (which I have also been asked): Wouldn't the gravitational pull of the Earth attract the denser material and hence displace the center of mass of the Moon toward itself? The answer to both of these questions is that the motion of the Moon is exactly in equilibrium with the gravitational pull of the Earth (as must be the case for an orbiting body), so it feels neither of these forces. If it moved around the Earth faster than its orbital velocity, it would experience a net "centrifugal" force, and if it moved more slowly, it would feel the Earth's gravity. A good example of that is us: We're standing on the Earth's surface and moving much more slowly than the orbital velocity at this distance, so we mainly feel the Earth's gravity. The "centrifugal" force is also felt in the form of weaker apparent gravity toward the equator; this effect results in the fattening of the Earth about the Equator (also known as oblateness). The main gravitational effect of the Earth on the Moon is tidal: The Earth's gravity is slightly weaker on the far side and stronger on the near side, which elongates the Moon along the Earth-Moon axis.