Physical Science:

The phases of the moon [Figure 12.2.1.(b)] refer to the appearance of the illuminated portion of the moon as seen from Earth as it orbits around our planet. The moon goes through a cycle of eight distinct phases:

Moon spins on its axis in about 27.3 Earth’s days. This time is known as sidereal month. A sidereal month is the time it takes for the Moon to complete one orbit around the Earth as measured relative to the fixed distant stars. This is slightly shorter than the time (29.53 days) it takes for the Moon to return to the same phase (such as full moon to full moon) as observed from Earth. This time is known as the synodic month or Lunar month. This is the same amount of time that the Moon takes to orbit around the Earth as seen from the moon. As a result, the same side of the Moon always faces the Earth, a phenomenon known as tidal locking. While the moon orbits a complete circle around the earth, the earth itself rotates around the sun. The difference between these two periods is due to the fact that the Earth is also orbiting around the Sun, so the Moon has to travel slightly farther to complete a full cycle of phases as seen from Earth.