Day and night

Activity 12.2 uses a globe as the Earth, with a small sticker on your own location. Looking down from above the North Pole, turn the globe anti-clockwise on its axis and watch your sticker go all the way round and back: one rotation.

Now use a torch as the Sun. In a fairly dark room, shine it on the globe from about 1.5 metres. Half the globe is lit and the other half stays dark.

The lit half is having day; the dark half is having night. A ball can only be lit on one side at a time — so it is always day somewhere and night somewhere else.

In India, the Sun rises first in the east and later in the west. Turn the globe one way and then the other while watching eastern India. The light reaches the eastern part first only when the globe turns from West to East — which is how the Earth really turns.

Watch your own sticker as the globe turns West to East. It goes through a cycle of day and night. Sunrise is when it moves into the light; sunset is when it moves into the dark.

So the Earth's rotation from West to East causes day and night. The side facing the Sun has daytime; the other side has night.

Imagine standing on the Equator for one rotation, facing North with your back to the South. In the morning the Sun appears on your right, in the East. Around noon you have been carried round until the Sun is overhead. By evening you have turned further, the Sun disappears on your left, in the West, and the stars come out.

In the mid-nineteenth century Léon Foucault used a long pendulum to give the first simple demonstration that the Earth rotates. A 22-metre Foucault pendulum hangs in the Constitution Hall of the new Parliament building in New Delhi.