The Moon takes about 27.3 days to complete one full orbit around the Earth relative to the fixed stars; this is called the sidereal period
. However, because the Earth is also orbiting the Sun, it takes about 29.5 days for the Moon to return to the same phase (e.g., from full moon to full moon), known as the synodic period or lunar month
. Additionally, the Moon is tidally locked with Earth, meaning it rotates on its axis once every 27.3 days-the same time it takes to orbit Earth-so the same side of the Moon always faces Earth
. In summary:
- Sidereal month (orbit relative to stars): ~27.3 days
- Synodic month (orbit relative to Sun/phase cycle): ~29.5 days
- Moon’s rotation period (spin on axis): ~27.3 days (tidally locked to orbit)
This explains why the Moon appears to take about one month to orbit Earth, with the difference between the two periods due to Earth's simultaneous orbit around the Sun
