### Does the moon have seasons?

Well it’s quite a simple question isn’t it? But can we actually work out the answer from what everyone knows about the moon, let’s have a quick look at the facts we all observe about the moon.

1. The part in the sun changes.
2. It’s always the same face that faces us.
3. It’s maximum height above the horizon changes.

And what do we need to know to decide whether the moon has seasons?

What is a season?

A reproducible change in weather patterns, or something more fundamental, my normal port of call in answering this type of question tends to be wikipedia, so let’s see what it says:

A season is a division of the year, marked by changes in weather.

Seasons result from the yearly revolution of the Earth around the Sun and the tilt of the Earth’s axis relative to the plane of revolution. In temperate and polar regions, the seasons are marked by changes in the intensity of sunlight that reaches the Earth’s surface, variations of which may cause animals to go into hibernation or to migrate, and plants to be dormant.

I think this leaves us nicely open to interpret a season as not requiring weather but being something more general in just requiring an off axis rotation of some form from the plane of orbit around the sun.  Before we all get too confused about what we’re talking about here, I think it’s time for a couple of diagrams and definitions.

The orbit of the Earth around the sun showing the rotation of the Earth.

The Moon orbits the Earth at around 5degrees to the Earth's orbit around the Sun.

We can see here where the Earth seasons come from. The tilt of the Earth means that different parts of the Earth are closer to the Sun at different times increasing the flux of light hitting the Earth.

The moon is tidely locked to the Earth.  This is important as it induces a rotation that is orthogonal to its rotation around the Earth.  As the Moons orbit around the Earth is at around 5degrees to the ecliptic (the plane of the Earth’s orbit around the Sun), so it’s axis of rotation is off axis to its orbit around the sun.  —   Please see the comments below, the Moons orbit is not orthogonal to its orbit.

From this quite simple analysis it can be said that yes the Moon does have seasons…

Before I move on to another post a quick note that moons orbit might be on the other side of the ecliptic, but the sentiment remains the same…

Wednesday, November 16th, 2011