Earth
The Moon goes around Earth at an average distance of . It is locked to Earth, so that it always has the same half facing Earth; the other half is called the "dark side of the moon". It takes about 27 days for the Moon to go all the way around Earth, but because Earth is moving around the Sun at the same time, it takes about 29 days for the Moon to go from dark to bright to dark again. This is where the word "month" came from, even though most months now have 30 or 31 days.