天平動

thumb|The lunar phases and librations in 2019 in the Northern Hemisphere at hourly intervals, with music, titles, and supplemental graphics alt=Over one lunar month more than half of the Moon's surface can be seen from the surface of the Earth.|thumbtime=0:02|thumb|Simulated views of the Moon over one month, demonstrating librations in latitude and [[longitude. Also visible are the different phases, and the variation in visual size caused by the variable distance from the Earth.]] thumb|Theoretical extent of visible lunar surface (in green) due to libration, compared to the extent of the visib