Let’s think about an instance with an imaginary planet. On this photo voltaic system, the planet completes one orbit round its solar in 8.6 photo voltaic days, as a substitute of 12 months, because the Earth does. (I am utilizing a shorter yr as a result of it magnifies the distinction between photo voltaic and stellar days, so you may see it extra simply.)

Right here is an animation exhibiting the distinction between photo voltaic and stellar days for this planet. The arrow exhibits when a sure spot on the planet factors at a distant star (which might be method outdoors the body) or at its solar. The moment when it factors on the solar is when the solar could be on the highest level within the sky for an observer on that spot.

Discover that for a stellar day, the planet does certainly make one full revolution—with a time of 0.648 “time items.” (I additionally made up imaginary items of time for this instance.) Nevertheless, at this level within the movement, the solar is not again to the identical spot within the planet’s sky, as a result of throughout that stellar day the planet moved. It takes 0.726 “time items” earlier than the arrow factors again to the solar. So, on this case, the photo voltaic day is slightly bit longer than a stellar day, identical to on Earth.

Is it doable for the photo voltaic day to be shorter than the stellar day? Yup. If the planet rotates in a course reverse to its orbital rotation, then this backward rotation will get the solar again to the very best level sooner. This is what that appears like:

Nevertheless, due to the way in which photo voltaic techniques kind, planets normally rotate in the identical course as their orbital movement. In our photo voltaic system, solely Venus rotates backward. (OK, Uranus rotates on its aspect—I’m undecided if that counts as backward.) However nonetheless, the purpose is {that a} photo voltaic day is completely different than a stellar day.

Modifications in a Photo voltaic Day

For our make-believe planet, the size of every photo voltaic day was the identical because the earlier photo voltaic day. On Earth, this is not true. The distinction is that our imaginary planet had a round orbit, and the Earth’s orbit is not completely round—it is shut, however not precise.

This is what the imaginary planet would appear to be with an elliptical orbit. Be aware: I am not exhibiting the rotation of the planet on its axis. As an alternative, I’ve a pink vector arrow to symbolize the planet’s velocity—the longer the arrow, the quicker the planet is transferring.

Discover that when the planet will get nearer to the solar, it accelerates. Then it slows down when it will get farther away. There are a few methods to elucidate this phenomenon, however I’ll use the thought of angular momentum.

To be trustworthy, the maths wanted to completely perceive angular momentum can get slightly ugly. So, as a substitute, I am simply going to elucidate this with a pleasant demonstration.