The Earth rotates

Rashmika, aged 12, cycles to school in Kanniyakumari. She notices the coconut trees' shadows are long in the morning and shorter in the afternoon, and decides it is because the Sun moves across the sky. But she also remembers that the Earth moves round the Sun. Which is it?

Activity 12.1 uses a merry-go-round. Sit on it facing outwards while someone turns it slowly anti-clockwise. The things around you seem to turn the other way, clockwise. Keep your eyes on a tree: it comes in from your left, slides across, and goes out on your right — though the tree has not moved at all.

Now look at the sky the same way. From the Earth, the Sun appears in the East, crosses the sky and sets in the West. Is the Sun really moving? Or is the Earth turning, so the Sun only seems to move?

The answer: the Sun seems to move because we watch it from the Earth, which is spinning. We are on the merry-go-round.

Think of a spinning top, a fan or a ball. The Earth spins, or rotates, on its own axis in the same way. Rotation is a motion in which every part of an object moves in circles round an imaginary line through it; that line is the axis of rotation. Nobody has drawn it on the Earth — we imagine it.

The Earth's axis passes through its North Pole and South Pole. The Earth completes one rotation in about 24 hours — one day.

Seen from above the North Pole, the Earth rotates anti-clockwise, that is, from West to East. That is why the Sun appears to travel the opposite way, East to West.

Ancient Indian astronomers worked this out too. Aryabhata, writing the Aryabhatiya around the fifth century CE, explained it with a picture: a person in a boat moving forward sees the still riverbank moving backwards; in the same way, the fixed stars seem to move towards the west. His value for one rotation — about 23 hours 56 minutes 4.1 seconds in today's units — is very close to the accepted value.