My last two posts have been on what exactly this thing is that we call a day. Did you notice that I never mentioned the rotation of the Earth? If I had asked you earlier to define a day, wouldn’t you have said it’s one rotation of the Earth?
Today nearly all of us think of the Earth as an object spinning in space. We can measure how long it takes to spin once if there is a fixed reference point out in space. Actually, there probably is no such thing because everything in the universe appears to move but that’s okay. Stars and galaxies are so far away from us that we can ignore how much they actually move in a day. We can treat them as fixed objects.
If the sky is clear tonight, you can go out and look for a bright star that just rose above a building or a tree. Then you can go out again tomorrow night and see if the star is at the same spot at the same time.
You should find that the star took slightly less than 24 hours to get back to the same spot in the sky. If a star rises at 10 pm tonight, it should rise around 9:56 pm tomorrow night.
A 360 degree rotation of the Earth does not take exactly 24 hours. It takes 23 hours and 56 minutes.
Why the four minute discrepancy? As the Earth spins, it does something else as well. It moves slightly in its orbit around the Sun. After completing one full rotation, the Earth has to turn for another four minutes before the Sun returns to the same location in the sky. This is shown in the figure below. Astronomers refer to our 24 hour day as a solar day. One rotation of the Earth is known as a sidereal day.
A few thousand years ago, there were a few people that claimed that the Earth rotates. But the majority seemed to believe it doesn’t. And you can’t blame them. We don’t feel the Earth move. We see objects moving around us.
Those who paid close attention noticed that the Sun appears to move slightly slower than the stars. The Moon moves even slower. It takes 24 hours and 50 minutes to go around the sky once.
If everything goes around us and we could define the day based on any of them, the obvious choice is the Sun. After all, we wouldn’t have daytime without it.
Even given what we know now, it makes more sense to define the day based on the Sun’s apparent motion than on the Earth’s actual motion. Otherwise the hours of the day would have nothing to do with daylight. The Sun would rise around 6 am one month, 8 am the following month, 10 am the month after that and so on until the time came when it rose at midnight. How strange would that be?