How Long Does it Take the Moon to Orbit the Earth?

Question posed by Elly.

The Moon is a popular subject on Blogstronomy so I was surprised to find that this question hadn't been covered before. We've had How Long is a Lunar Day? and although that's a different question it's also quite similar, and the quick answer is the same: "about a month". In fact the words "month" and "moon" share an origin in language.

It's not quite as simple as that, though*.

How do you tell when a full orbit is complete? To measure anything like an orbit (or a rotation) you need to be able to compare it to something: Hovering completely still above it, like you can imagine you're doing when looking at the animation below, would be great but in reality it would be very difficult. How would you know when you were completely still? 

Image: An animation of the Moon's orbit around the Earth
(I think it is to scale, but obviously speeded up quite a lot).
Phoenix7777 / CC BY-SA (https://creativecommons.org/licenses/by-sa/4.0)

There are different things we might compare the Moon to that might help us to see when an orbit starts or ends. Here are a few (most of which were recognised by the ancient Babylonians):

Phases of the Moon

Pick any point in the Moon's phases and see how long it takes to get back to that same point again. This is probably the most well-known way of measuring a Lunar orbit, and on average this takes about 29.5 days. You might wonder why there are other methods, and the answer is that this one can vary quite a bit from month to month. It's known as a synodic month.

Perigee to Perigee

Another method is to start timing when the Moon is at its closest to Earth and stop timing when it gets back to that point. This also varies but on average takes a just over 27.5 days. It is known as an anomalistic month.

Crossing the Plane

This is a bit like the previous one, but instead the clock is started when the Moon crosses the plane of the Earth's orbit around the Sun, and stopped when it gets back to that point. This is a draconic month and is about 27.2 days long.

Look to the Stars

Some of the problems** with the methods above come from the fact that they involve comparing things that are moving about with other things that are moving about. This method zooms out and uses the background stars as a fixed reference frame against which the motion of the Moon can be compared. This is called a sidereal month and takes about 27.32 days to complete.

What's Your Sign?

Over the course of a month (however you measure it) the Moon appears to move around the sky with respect to the background stars. A tropical month times the Moon from one point on this path until it gets back. To two decimal places it is the same length as a sidereal month, 27.32 days, but if you measure it really accurately it is actually a whopping 7 seconds shorter.

I think "about a month" will do in most cases, though.

Further Reading

• More about the Moon.

* It never is, is it?

** They're not really problems as such: it all depends on what you want to use them for. One of the things that people might want to use them for is to measure other things against. In this case it helps if you've got something that behaves pretty predictably rather than wobbling about all over the place (relatively speaking).