Lunar seismic activity detected by Apollo seismographs clearly shows a monthly periodicity. Using 36 months of data, Lammlein et al. (1974) found that the period is 27.2 days, the lunar nodical period. A search for such periodicity in earthquakes, based on the 8 years covered in the NOAA catalog, shows an analogous effect at half the lunar sidereal period, 13.65 days. The probability that the effect occurs randomly is assessed. The highest earthquake rate occurs at the two times each month when the moon crosses declination zero. A model based upon monthly changes in tidal forces explains the main patterns in both earthquake and moonquake smaples by emphasizing directional relationships rather than variation of earth-moon separation.