One of the strange quirks about the Moon is its orbit around the Earth. The Moon is tidally locked to the Earth. This means that it always presents the same face to the Earth, as it orbits around our planet. The Moon orbit period is the same as its rotation period: 27.3 days.
The Moon follows an elliptical path around the Earth. Its average orbital distance is 384,748 km. But it can get as close as 364,397 km, and then as distant as 406,731 km. That’s a big difference in distance to the Moon, and you can really see it from here on Earth, especially when the Moon is full. When it’s full and at its closest point, the Moon can look 10% bigger, and 30% brighter than when it’s at a more distant point in its orbit.
There are a few ways to consider a lunar month, the amount of time it takes for the Moon to go around the Earth. If you could look from overhead at the Earth-moon system, you would see the Moon takes 27.3 days to completely orbit the Earth compared to the background stars. This is called a sidereal month.
Another way to look at it is how long it takes for the Moon to return to the same phase; how long it takes to go from full moon to full moon. This is called a synodic month, and it takes 29.5 days. That’s because the Earth is also orbiting around the Sun, and so it takes the Moon two extra days to make catch up to the same angle of illumination from the Sun.
The Moon is slowly drifting away from the Earth at a rate of about 1 cm per year. Eventually, the Earth and the Moon will be tidally locked to each other, so the same side of the Earth will always face the Moon, just like the same side of the Moon always presents the same face to the Earth. But this won’t happen for billions of years from now.
We’ve written many articles about the Moon for Universe Today. Here’s an article about lunar orbit, and how the Moon doesn’t orbit the Sun.
We’ve also recorded an episode of Astronomy Cast all about the Moon. Listen here, Episode 113: The Moon, Part 1.