The correct way to express speed of rotating object

By MUNGAI KIHANYA

The Sunday Nation

Nairobi,

01 September 2024

A secondary school student asked me recently how fast the earth rotates about its axis. I told her that the answer depends on where the measurement is done and it can be anything between zero and about 1,667km/h. This came as a surprise to her because she expected a single answer – after all, “the earth is a solid object”.

How can different parts of a solid be moving at different speeds? Well, the planet rotates about an axis that runs from one pole to the other. So, if you are at the pole, you won’t be going anywhere – you will be spinning at the same spot. Your speed is then zero km/h.

If you move away from the pole, you will start moving around in a circle that is centred at the pole. The farther you are from the pole, the larger the circle becomes. But it does not increase in size indefinitely: it reaches its maximum when you get to the equator. After that it starts reducing and goes to zero again when you reach the opposite pole.

For this reason, the people who live at (or near) the equator move the fastest and those near the poles are the slowest. The diameter of the earth at the equator is about 12,800km, thus, the distance travelled in one rotation is about 40,000km. This translates to a speed of about 1,667km/h.

For the residents of London, UK, their distance from the axis of rotation is smaller than that of the equator. Knowing the geographical latitude of the city (51.5 degrees north) and some basic trigonometry, a secondary school pupil should be able to calculate its distance from the axis of rotation.

This comes to about 4,000km – much nearer than the 6,400km (a half of the diameter) at equator. Consequently, London travels a total distance of about 25,000km in one day. This works out to a speed of slightly over 1,000km/h which is about 60 per cent of that at the equator.

Because of this big variation in the values, it is more convenient to state the speeds of rotating objects in degrees per hour instead of km/h. One complete rotation is 360 degrees. Whether you are near the axis or a great distance away, you will traverse over 360 degrees. Thus, the rotational speed of the earth is 15 degrees per hour. This figure is the same everywhere one the planet – even inside it!