The time between a full moon and a new moon is approximately 29.5 days. This is known as a synodic month.

Here's why:

* The moon orbits the Earth, and it takes about 27.3 days to complete one orbit (this is called a sidereal month).

* However, during this time, the Earth is also moving in its orbit around the sun. This means that the moon needs to travel a bit further to catch up to the sun and appear in the same position relative to the sun and Earth again.

* This extra time is why the synodic month (full moon to full moon or new moon to new moon) is slightly longer than the sidereal month.