Re: I had to look that one up....
I remember it because I was a sppace cadet at the time of Apollo. Folks were mildly surprised to realized that the Newtonian point gravitational math model didn't work for the moon and that the mass concentrations, likely from ancient nickel-iron impactors made slight but detectable and significant modifications. Point is, you can't STAY in a really low lunar orbit because of it.