How Long is a Lunar Day?

"How long is a lunar day - a day on the moon?" - Question posed by Storm.

First question: what is a day? Check out this post for that.

If we wanted to measure the length of a lunar day - that is a day on the Moon - we might think of timing how long it takes for the Moon to rotate with respect to the body it's orbiting around: the Earth. If we started that experiment, though, we'd be there for a while. The Moon points the same face towards the Earth constantly, so if we used this to define its day we could argue that it was eternal!

We're more likely to want to define it with reference to the Sun, however, because sunrise and sunset are important things for a day to have (in my opinion). Given that the man in the Moon is always looking at Earth, the Moon takes the same amount of time to rotate with respect to the Sun as it does to orbit the Earth. This means that a day on the Moon lasts about 27 Earth days, 7 hours and 43.2 minutes.

Except... during this time the Earth has moved a bit further along its orbit, so the Moon needs to rotate a bit further before the same point on its surface returns to experience its 'noon'. This means that a lunar day is actually 29 (Earth) days, 12 hours, 44 minutes and 3 seconds long. As with the discussion of the length of an Earth day in the last post, this figure is not static due to things like the eccentricity of the Earth-Moon system's orbit around the Sun.

So, a quick answer to the original question, "how long is a lunar day?" is "about a month".

Except it's never that simple: the term 'lunar day' is sometimes used to refer to the length of time as measured on Earth between successive moonrises at a given location. This time, due to the fact that the Moon orbits in the same direction as the Earth rotates, is about 24 hours and 50 minutes.

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