> Gravitational anomalies slightly distorting the orbits of some Lunar Orbiters led to the discovery of mass concentrations (dubbed mascons) beneath the lunar surface caused by large impacting bodies at some remote time in the past. These anomalies are of sufficient magnitude to cause a lunar orbit to change significantly over the course of several days. They can cause a plumb bob to hang about a third of a degree off vertical, pointing toward the mascon, and increase the force of gravity by one-half percent.
> ...
> Study of the mascons' effect on lunar spacecraft led to the discovery in 2001 of "frozen orbits" occurring at four orbital inclinations: 27°, 50°, 76°, and 86°, in which a spacecraft can stay in a low orbit indefinitely.
Consider that - there are only four inclinations where you an orbit the moon without needing to do constant adjustments. Whereas with the Earth basically all the inclinations are available.
