The Size Distribution of Lunar Craters

All the readily visible crater diameters have been measured on four Ranger 7 pictures, four Ranger 8 pictures, five Ranger 9 pictures, and on three areas of the H. P. Wilkins map of the Moon. The total of 1600 measurements, ranging from 0.65 to 69 000 metres, have been used to establish the size distribution of craters on three areas of the Moon. The results show that in all three areas the frequency of crater occurrence is inversely proportional to the square of the crater diameter, and that this relation holds accurately over the whole size range. The overall crater frequency is shown to be lowest in the Mare Tranquillitatis, 1.4 times higher in the Mare Nubium, and 2.5 times greater in the Alphonsus region. The consequences of extrapolating this distribution law to still larger and smaller structures is briefly examined.