Could climate change change the length of the day?

Let’s consider an example with an imaginary planet. In this solar system, the planet orbits its sun in 8.6 solar days instead of 365 days as Earth does. (I use a shorter year because it increases the difference between solar and sidereal days so you can see it more easily.)

Here is an animation showing the difference between solar and sidereal days for this planet. The arrow indicates when a specific point on the planet points to a distant star (which would be path outside the frame) or at its sun. The moment it points to the sun is when the sun would be at the highest point in the sky for an observer at that point.

Video: Rhett Allain

Note that the planet actually makes one complete revolution for one sidereal day – with a time of 0.648 “time units”. (I also invented imaginary units of time for this example.) At this point in the motion, however, the Sun is not back in the same spot in the planet’s sky because the planet was moving on that sidereal day. It takes 0.726 “time units” for the arrow to point to the sun again. So in this case, the solar day is slightly longer than a sidereal day, just like on Earth.

Is it possible that is sunny day? shorter than the starry day? Yes indeed. If the planet rotates in a direction opposite to its orbit, this reverse rotation brings the sun back to the highest point sooner. This is what it looks like:

Video: Rhett Allain

However, due to the way solar systems form, planets usually spin in the same direction as their orbit. In our solar system, only Venus rotates backwards. (OK, Uranus is spinning on its side – I’m not sure if that counts as backwards.) But the point is, a solar day is different than a sidereal day.

Changes on a sunny day

For our phantom planet, the length of each solar day was the same as that of the previous solar day. On earth that is not true. The difference is that our imaginary planet had a circular orbit, and Earth’s orbit isn’t perfectly circular – it’s close, but not exact.

This is what the imaginary planet would look like with an elliptical orbit. Note: I do not show the rotation of the planet around its axis. Instead, I have a red vector arrow representing the planet’s speed – the longer the arrow, the faster the planet is moving.

Video: Rhett Allain

Note that the planet speeds up as it gets closer to the sun. Then it slows down when it’s farther away. There are a couple of ways to explain this phenomenon, but I’ll use the idea of ‚Äč‚Äčangular momentum.

To be honest, the math required to fully understand angular momentum can get a little ugly. Instead, I’ll just explain this with a nice demonstration.

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