Calculating the speed of the Earth

By MUNGAI KIHANYA

The Sunday Nation

Nairobi,

05 September 2021

A reader who prefers anonymity asks a rather straightforward question: “How fast is the Earth moving and why don’t we feel it?” Now; we know that it takes one year to go round the sun; that is, approximately 365.25 days. Each day is approximately 24 hours, thus it takes about 8,766 hours to make one complete revolution.

The Earth follows an almost circular path at an average distance of about 150 million kilometres from the centre. Therefore, the journey around the sun is a total of about 3.14 x 2 x 150 million km; That is 942 million km.

This distance is covered in 8,766 hours, so the average speed is about 107,000km/h. This is very fast: in the three or so minutes that it will take you to read this article, the planet will have traversed over more than 5,000km!

With that knowledge, the next question follows naturally: why don’t we feel this motion? Why don’t feel the gust of wind blowing past? The answer is that the planet moves with everything on it – people, plants, the atmosphere etc. And since there are no potholes along the way, we can’t tell that we are moving.

The only way to know if we are moving or not is by looking “outside” the Earth; at the stars in the sky and observing carefully how they change their position as the year progresses. But, of course, that brings up another question: is it the Earth that is moving or the stars?

For many centuries, people debated over this question – some were sent to life imprisonment and others even killed for supporting the wrong side! Eventually, when distances to stars were measured with fair accuracy, it turned out that the only way the observed movements of stars and planets can happen consistently is if the Earth was also moving around the sun.

Going back to the calculation of the speed of the Earth around the sun, you may have noticed that I used the word “approximately” in reference to the 24 hours in a day. Yet we use clocks that measure exactly 24h each day, so where does the approximation come from? Well, the truth is that the Earth takes 23h, 56m and 4.0905 seconds. That is a whole 4 minutes shorter than 24h!