* Sidereal Month: The time it takes the Moon to complete one full orbit around the Earth relative to the stars is called a sidereal month. This takes about 27.3 days.
* Synodic Month: The time it takes the Moon to return to the same phase (like a full moon) is called a synodic month. This is what we usually think of as a month, and it takes about 29.5 days.
Why the difference? The Earth is also moving around the Sun while the Moon orbits. So, for the Moon to appear in the same position relative to the Sun (and thus have the same phase), it needs to travel a bit further than just one full orbit around Earth.
