Does the Earth rotate once per day? Not really…

In  7th grade physics, we learn that the Earth takes 23 hours and 56 minutes for one rotation. Like me, have you wondered why  our day is exactly 24 hours instead of this precise figure of 23hr 56min?. Did we just round it up?  You may think it is negligible. Wouldn’t this shift of 4 minutes compound and cause a noticeable shift between the time shown on our clocks and the “actual time”.

A simple calculation reveals that after one month there would be a shift of two hours, which means that 6 months later, there will be a shift of 12 hours and day becomes night and night becomes day!  So, there must be more to this story which I set out to investigate.

While watching an animation of the Earth revolving around the Sun, I noticed how the terminator (the line that separates day and night on the Earth) moves. Watching the terminator rotate once a year made me realize that this might be connected to the length of a day. As the Earth’s rotation and revolution are in the same direction, the terminator also moves in the same direction as the rotation. This means that a person standing on Earth will have to wait slightly longer than one rotation for the Sun to return to the same position in the sky.


The image illustrates how the Earth has to rotate a bit more than 360 degrees for the Sun to be overhead during noon the nest day. It turns out that the Earth needs 4 extra minutes to rotate this extra bit and that is why the length of one day is exactly 24 hours. 

The 24-hour definition of a day is called a “solar day” and is the time taken by the Sun to return to the same position in the sky. The time taken for Earth to rotate once on its axis (relative to faraway background stars) is called a “sidereal day”. So even though there are 365 (solar) days in a normal year, the Earth actually rotates 366 times (sidereal days). During leap years, the Earth rotates 367 times, even though there are 366 days. One day is “lost” every year as the Earth has to rotate a little extra every day to “catch up” with the Sun. If the Earth was revolving around the Sun in the opposite direction, we would instead gain one day, and there would be 367 days in a year, with each solar day being 23 hours and 52 minutes long. But that’s a story for another time!


Image credits:

Williams, M. (2016, March 25). Sidereal Day vs. Solar Day [Digital image]. Retrieved September 26, 2020, from https://www.universetoday.com/47181/earths-rotation/

Location: Bengaluru, Karnataka, India

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