Orbit of the Moon

The Moon takes 27.3 days (3.9 weeks) to orbit around the Earth. One side of the Moon faces the Earth at all times, so the moon is tidally locked to Earth.
The phases of the Moon come from the position of the Moon relative to the Earth and Sun. If the Moon and Sun are on opposite sides of the Earth, there is a full moon. If they are on the same side of Earth, there is a new moon.
The Moon makes one circle around Earth, called the "sidereal month", in 27.3 days. But in this time, the Earth continues to move forward, so the Moon will be in a different place relative to the Sun, and the phase of the Moon will be different. To reach the same phase, the Moon must travel a bit farther. The time for the Moon to reach the same phase, called the "synodic month", is 29.5 days.