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Will climate change change the length of the day?

Let’s consider an example of an imaginary planet. In this solar system, the planet completes one orbit around the sun in 8.6 solar days, rather than the 365 days that Earth does. (I use the shorter year because it magnifies the difference between the sun and sidereal days so you can see it more easily.)

This is a display that shows the sun and the stars The difference between the animation of the planet’s days. The arrows show when a point on the planet points to a distant star (which would be the out-of-frame way) or its sun. The moment it points towards the sun, the sun will be at its highest point in the sky for observers at that location.

Video: Rhett Allain

Note that for a sidereal day, the planet does make one full revolution – 0.648 “time units”. (I also made up imaginary units of time for this example.) However, at this point in the motion, the sun doesn’t return to the same place in the planetary sky because the planets moved during that sidereal day. It takes 0.726 “time units” before the arrow points to the sun. So, in this case, the solar day is a little longer than the sidereal day, just like on Earth.

It is possible that the solar day is shorter than the sidereal day ? Yes. If a planet spins in the opposite direction of its revolution, that reverse rotation will bring the Sun back to its highest point more quickly. It looks like this:

Video: Rhett Allain

However, due to the way the solar system was formed, planets usually move in the same direction as their orbits. In our solar system, only Venus rotates backwards. (Well, Uranus is spinning on one side – I’m not sure if that counts as an inversion.) But, the point is that a solar day is not the same as a sidereal day.

Variation in one solar day

For our fictional planet, each solar day is the same length as the previous solar day. On Earth, this is not true. The difference is that our imagined planet has a circular orbit, while Earth’s orbit is not perfectly circular – it’s close, but not precise.

This is what an imaginary planet would look like in an elliptical orbit. Note: I’m not showing the rotation of the planet on its axis. Instead, I have a red vector arrow to represent the speed of the planet – the longer the arrow, the faster the planet moves.

Video: Rhett Allain

Note that as the Earth gets closer to the Sun, it accelerates. Then it slows down as it gets further away. There are several ways to explain this phenomenon, but I will use the concept of angular momentum.

To be honest, the math required to fully understand angular momentum can get a bit ugly. So, instead, I’ll explain this with a nice demo.

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