Parallax, in the case of the Moon, is caused by the difference in location of the observer as the Earth turns, since over the course of an evening, the Earth has rotated halfway though its full rotation, and therefore the position of the observer is rather markedly moved. makingupcodes.com

The means that the apparent position of the Moon will shift slightly as the position of the observer shifts slightly.

The parallax is also determined by the distance of the Moon, so to find the angle of parallax, we need to know the parallax at perigee, and then we can use a similar formula to find the angular diameter
Parallax = (Parallax at perigee) * (Semi-major axis) / (Distance)

		public static void CalcMoonParallax(DateTime dDate, DateTime dEpoch, double fMEpochLong, double fMPeriLong, double fMAscNode, double fMIncl, double fMEcc, double fSEpochEclLong, double fSPeriEclLong, double fSEcc, double fMSMA, double fVParallax, ref double fMParallax)
		{
			double fRho;

			fRho = 0;
			CalcMoonDistance(dDate, dEpoch, fMEpochLong, fMPeriLong, fMAscNode, fMIncl, fMEcc, fSEpochEclLong, fSPeriEclLong, fSEcc, fMSMA, ref fRho);

			fMParallax = (fVParallax * fMSMA) / fRho;
		}
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