Let’s take an example with an imaginary planet. In this solar system, the planet completes an orbit around its sun in 8.6 solar days, instead of 365 days, as Earth does. (I use a shorter year because it amplifies the difference between solar and stellar days, so you can see it more easily.)
Here is an animation showing the difference between solar and stellar days for this planet. The arrow shows when a certain point on the planet points to a distant star (which would be way out of frame) or in its sun. The instant it points toward the sun is when the sun would be at the highest point in the sky for an observer at that location.
Note that for one stellar day, the planet indeed makes a complete revolution, with a time of 0.648 “time units”. (I also invented imaginary time units for this example.) However, at this point in the motion, the sun has not returned to the same spot in the planet’s sky, because during that stellar day, the planet s is moved. It takes 0.726 “time units” before the arrow points to the sun. So in this case, the solar day is a bit longer than a stellar day, just like on Earth.
Is it possible that the solar day is shorter that stellar day? Yeah. If the planet rotates in a direction opposite to its orbital rotation, this backward rotation will bring the sun back to the highest point earlier. Here’s what it looks like:
However, due to the way solar systems form, planets generally rotate in the same direction as their orbital motion. In our solar system, only Venus rotates backwards. (OK, Uranus spins sideways – not sure if that counts as a pullback.) But the thing is, a solar day is different than a stellar day.
Changes in a solar day
For our imaginary planet, the length of each solar day was the same as that of the previous solar day. On Earth, this 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 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 is moving.
Notice that as the planet gets closer to the sun, it speeds up. Then he slows down as he walks away. There are several ways to explain this phenomenon, but I will use the idea of angular momentum.
To be honest, the calculations needed to fully understand angular momentum can get a little ugly. So instead, I’m just going to explain that with a nice demo.