The
lunar geological timescale (or
selenological timescale) divides the history of
Earth's
Moon into five generally recognized periods: the
Copernican,
Eratosthenian, Imbrian (
Late and
Early epochs),
Nectarian, and
Pre-Nectarian. The boundaries of this time scale are related to large
impact events that have modified the
lunar surface, changes in
crater form that occur through time, and the size-frequency distribution of craters superposed on geological units. The absolute ages for these periods have been constrained by
radiometric dating of samples obtained from the lunar surface. However, there is still much debate concerning the ages of certain key events, because correlating lunar
regolith samples with geological units on the Moon is difficult, and most lunar radiometric ages have been highly affected by an intense history of bombardment.