A day at the poles is six months long!

By MUNGAI KIHANYA

The Sunday Nation

Nairobi,

03 April 2005

So; it is now clear why people living in the temperate countries (outside the tropics) change their clocks with the seasons – it’s because summer days are very long while winter days are short. But what causes these seasonal variations in the number of daylight hours?

To understand, let’s start from a basic point: the sun rises from the east and sets in the west because the Earth is rotating (from West to East). The axis of this rotation is a straight line running from the North Pole, through the centre of the planet to the South Pole. That is easy to comprehend if you live near the equator – like we do in Kenya. But if your home is located at one of the poles, your view of the motion of the sun is very different.

First of all, from the poles, there is neither East nor west. If you stand on the North Pole, for example, any direction you face will be south! From where, then, does the sun rise and to where does is it set?

Let’s leave the sun for a while and consider the stars. In Kenya, which is near the equator, we see them traverse the sky from East to West every night. But if you were at the North Pole and you looked up at night, you would see the stars moving in a circular path in the sky, never setting under the horizon! This is because the pole is on the axis of rotation of the Earth.

If you find that hard to picture in your mind, take a moment and close your eyes…do you see it now? If that is how the stars will appear to move, what about the sun? Indeed, when is it daytime and when is it night at the poles?

Since the axis of rotation of the Earth is slanted at 23.5 degrees to the axis of revolution, then during certain months, one pole is permanently exposed to the sun and at other times it is constantly facing away. Thus in the sunny season (polar summer), the sun rises and stays in the sky until the Earth has revolved to a position where the pole is in the dark.

Now, since the Earth takes one year to complete one revolution, then for about six months one pole will be permanently illuminated and the rest of the year it will be in darkness.  When the sun is visible in the polar sky, it also appears to move in a circular path (like the stars) near the horizon, completing one cycle every 24 hours, but never setting.

As we move away from the poles, the length of a midsummer day gradually shortens fro six months to 12 hours at the equator. Thus, in some parts of Europe, summer day can be more than 18 hours long. Easy, isn’t it?