Sidereal Day Vs. Synodic Day

I’ve talked here before about sidereal days and synodic days. What exactly is the difference between the two?

It all comes down to one thing: the object that you are using as a reference. For example, the sidereal day of the Earth is equal to 23 hours, 56 minutes, and 4 seconds. The sidereal day uses a distant star (which is basically any star that is not the Sun) to measure against. You would determine how long a star would take to reach the same location in the sky on consecutive days. Another way to look at it, is the length of time it takes the Earth to rotate 360°.

However, the synodic day is 24 hours. The best way to think of the synodic day is to just determine the length of time between two consecutive noons. A synodic day means the Earth rotates more than 360° to get the Sun in the same location in the sky. The reason this happens is because as the Earth rotates on its axis, it is also moving around the Sun. Granted, it is only moving a little less than a degree in its orbit (since the Earth year is about 365.25 days), but that is long enough that the Earth has to rotate just a tad more to get the Sun back to noon.

 

Solar Non-Uniform Rotation

Remember in the last post, the Sun is described as a "miasma of incandescent plasma"? In order words, it isn't solid.

We have this large ball of plasma that rotates. But because it isn't solid, different latitudes orbit at different rates.

Imagine we have a long rope that can withstand the immense heat of the Sun. We tie either end to the rotational axis on either pole and attach the rope to the Sun's surface.. We have something like this.

 

We allow the Sun to rotate, and we begin to notice something.  The rope nearest the equator is moving faster than the rope nearer the poles.  Now the rope looks like this.

 

How do we know this happens? We are able to observe the sunspots on the Sun and can see those spots near the equator are farther ahead compared to the spots in higher latitudes.  By measuring the speed of the spots, scientists were able to determine that the Sun takes about 30 days to rotate near the poles and only about 25 days at the equator.

We also notice the same differential rotation on the gas giants as well.