Steve’s Virtual Planet

How long is a month?

If you check deeper into the meaning of “month”, you quickly find Earth’s moon right at the center of the explanation. It turns out that the ancient Babylonians pretty much started this examination in the years around 500BC. These early mathematicians and astronomers took extra care in watching and noting the movement of the moon. They came up with several methods and terms to show how the moon tracked around the Earth and how long it took.

There are different ways to describe Earth’s orbit and the orbit of the moon around our planet. Its related to the geometry and timing of those orbits. The method for describing the complete cycle of the moon phases is known as the synodic month. This is the view of the moon from Earth’s perspective and is relative to our point of view. The other method has a more general and wider viewpoint and it is known as the siderial month.

In order to get a better feel for these two concepts, it’s handy to have some kind of diagram or, even better, an animation to show the Moon phases as it orbits the Earth. If you can get your mind around the geometry from your perspective and the solar system perspective (as in the animation), it’s easier to see how “the month” is defined in both of the methods above.

Ultimately, we are talking about lunar calendar systems and there are actually quite a few. The ancient astronomers weren’t just moon gazing but trying to understand what they were looking at. Their persistence lead to what is now a very refined but kind of crazy way of showing what month it is. After all, we want everyone to know our Birthday in time or maybe we just want to take a month off!