Figure 1-21 Why the Moon Goes Through Phases This figure shows 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.