The correct way to measure the rotation of the earth on its axis

By MUNGAI KIHANYA

The Sunday Nation

Nairobi,

09 March 2025

Samuel Mwaniki is curious about New Year celebrations. He asks “Given that New Year's Day is essentially a celebration of the completion of a revolution of the planet earth round the sun; and that a day is the completion of a rotation by the earth on its axis; and given the speed at which the earth travels around the sun; why do different parts of the planet celebrate New Year's Day at different times? One would imagine that the whole earth completes a revolution simultaneously and therefore everyone should celebrate that moment at the same time.”

Well, Samuel, the reason is that different parts of the world start tracking the revolution around the sun when the Earth is at different locations along the orbit. So, they finish and re-start at different times. This stems from the fact days are based on the rotation of the planet on its axis while years are the revolutions around the sun. These two motions are not synchronised; they do not depend on one another.

In fact, what we call one day is NOT the time it takes the earth to rotate once. One day is 24 hours exactly, but one rotation takes 23 hours, 56 minutes and 4 seconds. There is a 4-minute difference between the length of a day and the duration of one rotation!

The reason for this difference is that a day is based on the APPARENT movement of the sun as viewed from the earth and yet the earth also moves around the sun. To get the actual duration of rotation, we need to use a different reference point; different star which the earth is not revolving around. Any other star will do since they are all so far away – the nearest one is 40 trillion km from here.

It turns out that, while it takes the sun 24 hours from sunrise one day to sunrise on the next one, when we observe the same apparent motion for a different star, the duration is 23 hours 56 minutes and 4 seconds. This is a simple astronomical observation that anyone can do for themselves. You don’t even need a telescope! But you need make your observation very carefully so as to identify one star among the many that visible with the naked eye.

In this dry season, the night sky is quite clear so readers can check for themselves if what I have written here is correct!