Amount of illumination by sunlight of a planetary body as observed from Earth. The Moon goes through a cycle of phases that repeats every 29.531 days (synodic month). We see these phase changes occur with the Moon rising between 20-70 minutes later each day.
A complete cycle of phases is observable for the inferior planets, Mercury and Venus. Observations of the phases of Venus by Galileo were used to support the Copernican model of the The Sun and set of objects orbiting around it including planets and their moons and rings, asteroids, comets, and meteoroids..
Some or all content above used with permission from J. H. Wittke.