A sidereal month is the time it takes for the Moon to complete one full orbit around the Earth relative to the background stars. This period lasts about 27.3 days. It is measured from one position of the Moon in relation to a fixed star to the same position again. This is different from a synodic month, which is about 29.5 days and is based on the Moon's phases, such as from one full moon to the next. The difference arises because the Earth is also moving in its orbit around the Sun, requiring the Moon to travel a bit further to catch up with the changing position of the Sun.