I’ve researched it but it’s exact purpose eludes me.
In: 5
A sidereal day is the time it takes for the Earth to rotate once, about 23 hours, 56 minutes. A solar day is the time it takes for the Sun to return to the same position (well, same east-west position) in the sky, about 24 hours.
To see the difference, let’s imagine the Earth orbited the Sun faster than it does. Let’s make the year four (sidereal) days long.
Let’s picture the Earth orbiting and rotating as though both were in the same flat plane. Imagine a drawing with the Earth above the Sun, and you living on the side of the Earth facing the sun (the “bottom” of the Earth in this drawing). Both the orbit of the Earth, and the rotation of the Earth, are counterclockwise in this drawing, so at this moment, you are being carried to the right by the Earth’s rotation and the Earth is moving to the left in its orbit. It is currently noon in your local solar time, because the Sun is directly overhead.
Now, progress the Earth around by 1/8 of its orbit. It’s now up and to the left of the Sun. 1/8 of an orbit is 1/2 of a (sidereal) day, so the Earth’s rotation has carried you halfway around; you are now on the “top” of the Earth. But you are *not* on the opposite side of the Earth from the Sun – that is, it is *not* [solar] midnight for you right now! If the Earth had stayed put, it would be, but the movement of the Earth in its orbit has moved the position of the Sun relative to the Earth.
Progress the Earth around by another 1/8 of its orbit. It’s now to the left of the Sun. 2/8 = 1/4 of an orbit is 1 full sidereal day, so you’re once again on the “bottom” of the Earth, where you started. But now that the Sun is to the *right* of Earth, it’s not noon for you: it’s dawn! The sun is just rising over your horizon. 1 full sidereal day has passed, but you haven’t yet seen 1 full *solar* day, because the Earth’s orbit is effectively delaying the speed at which the Sun’s apparent position in your sky changes.
It turns out that the total effect of this is that you have 1 less solar day than sidereal day every year*. In our example, there are 4 sidereal days per year, so there are 3 solar days per year. On the real Earth, there are 36**6**.24 sidereal days per year, and thus 365.24 solar days per year (which is what you think of as “the number of days in a year”, because if we say “day”, we usually mean “solar day”). The difference in length (365.24/366.24 = 0.9973) corresponds to the four-minute difference in the time each type of day takes (0.9973 * 24 hours = 23.93 hours = ~23 hours, 56 minutes).
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* provided both rotations are in the same direction, which is true for Earth. If the rotations are in opposite directions, you get one *more* solar day than sidereal day.
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