Rotation and revolution of the Earth

Does the Sun move, or the Earth?

Every day the Sun rises in the East, crosses the sky and sets in the West. It is natural to think the Sun is travelling. But in Class 6 you learnt that the Earth moves. So which is it? A ride on a merry-go-round gives the answer.

Lessons from a merry-go-round

Sit on a merry-go-round facing outwards and let someone turn it slowly anti-clockwise. The trees and buildings around you seem to swing past the other way, clockwise. A tree you are watching appears from your left, passes in front of you and disappears on your right, though it never moved. The Sun behaves the same way. It seems to move from East to West because we watch it from an Earth that is turning.

The spinning Earth

Like a top, a fan or a spinning ball, the Earth rotates on its own axis: every part moves in a circle around an imaginary line through it. The Earth’s axis passes through the geographic North Pole and South Pole, and one rotation takes about 24 hours. Seen from above the North Pole, the Earth turns anti-clockwise, that is, from West to East.

Day and night

In a dark room, shine a torch on a globe from about 1.5 metres away. Only the half facing the torch is lit; the other half is dark. On the Earth, the lit half has day and the dark half has night. Turn the globe from West to East and watch a sticker marking your town: it moves into the light (sunrise), faces the torch (noon), moves into darkness (sunset), and comes round again. That is why eastern India sees the sunrise first. To someone on the Equator, the Sun appears in the East in the morning, is overhead around noon and sinks in the West in the evening, after which the stars come out.

The turning sky

The stars too seem to rise in the East and set in the West. Watch the Big Dipper, Saptarishi, every two hours on one evening between March and May: it slowly circles round the Pole Star, Dhruva Tara. The Pole Star hardly moves, because the Earth’s axis points almost straight at it. Cameras left open for a long time record the stars as circular arcs, called star trails. The Moon, too, comes up in the East and goes down in the West, for the same reason.

Aryabhata, around the 5th century CE, explained this in his Aryabhatiya: someone in a boat moving ahead sees the still bank slide backwards, and in the same way the fixed stars seem to move west. His value for one rotation, about 23 hours 56 minutes 4.1 seconds, is remarkably close to today’s. In the 19th century Leon Foucault showed the rotation with a long pendulum, and a 22-metre Foucault pendulum now hangs in India’s new Parliament building.

Going round the Sun

While it rotates, the Earth also revolves around the Sun. Its path, the orbit, is nearly a circle when seen from above, and one revolution takes about 365 days and 6 hours. As the Earth moves along its orbit, its night side looks out at a different part of space, so the stars that rise at sunset change from month to month. The Bhil and Pawara communities of the Tapi valley watched for certain stars to know that the monsoon was coming.

Why it matters

Rotation sets the rhythm of every day, and revolution the rhythm of the year. See how revolution and the tilted axis give us seasons in seasons on the Earth, and read about solar and lunar eclipses. Answers to the textbook questions are in the NCERT solutions for Chapter 12; see all of Chapter 12 notes.