A day is not the time the earth takes to rotate once!

By MUNGAI KIHANYA

The Sunday Nation

Nairobi,

17 January 2021

What is a day? Is it 24 hours? Well, while it is true that a day is divided into 24 equal segments of time (hours), it would be wrong to then define a day as 24 hours. The question here is: what is that duration that has been divided into 24 fractions?

A day is the time it takes the sun to traverse across the sky and return to the same place where it started. This is the duration that is divided into 24 hours.

It is very easy to track the movement of the sun across the sky. We used to do it when I was in primary school. The roof in our classroom had a small hole through which a beam of sunlight would pass and form a dot, about one centimetre in diameter, on the floor.

We would monitor the positions of this dot using the cracks on the floor that corresponded with important bell rings – lunch time and end of school. This way, we were able to tell when to start getting ready to leave the classroom – just before the bell rings.

This timing system was quite accurate. Ancient human civilisations applied the same idea in making the earliest clocks. They tracked the movement of the shadow of a pole permanently fixed to the ground. Soon enough, they realised that they could also use the same pole to track the movement of stars in the night sky.

 When mechanical timers – water clocks, hourglasses etc – were developed, the ancient people noticed a discrepancy: the duration the sun takes to return to its starting point is slightly longer than what a star takes to go through a similar cycle.

Today, it is known that, while the sun takes 24 hours to complete its journey across the sky, the stars take 23 hours, 56 minutes and 4 seconds. If you time a star, you will notice that, after 24 hours, it will have advanced by about one degree westward from where it was the previous day.

This gradual advancement continues day-by-day and eventually the star disappears from view. However, after one year, it returns back to the starting point at the same time at night.

This strange behaviour of stars perplexed human civilisations the world over for many centuries – nay, for millennia. Eventually, the ancients concluded that the stars were revolving around their home planet. But, of course, we now know that they were wrong!