Figure 3-2: Why the Moon Goes Through Phases This figure illustrates the Moon at eight positions on its orbit, along with photographs of what the Moon looks like at each position as seen from Earth. The changes in phase occur because light from the Sun illuminates one half of the Moon, and as the Moon orbits Earth we see varying amounts of the Moon’s illuminated half. It takes about 29½ days for the Moon to go through a complete cycle of phases, and each of the phases shown is visible for one day during the complete cycle. The times given in the figure for each phase correspond to the time each phase is observed highest in the sky, halfway between its rise and set.
(Photographs from Larry Landolfi/Science Source)