The Earth's revolution

The stars you see in the East just after sunset are not the same all year; they change over the months. Why?

While it rotates on its axis, the Earth also revolves round the Sun. These are two different motions:

• Rotation: spinning on its own axis (one turn ≈ 24 hours).
• Revolution: moving around another object (one trip round the Sun).

The path an object follows while revolving round another is its orbit.

Seen from above, the Earth's orbit round the Sun is nearly a circle. In the Grade 6 picture of the Solar System it looked stretched out, but only because that picture showed the orbit from the side.

The Earth goes once round the Sun in about 365 days and 6 hours — one year.

Now the answer. Every evening the Sun sets and the night sky appears; that is because of rotation. But because the Earth is also moving round the Sun all the time, our night side faces a slightly different direction in space each month. So the stars we see after sunset slowly change through the year.

You can see this yourself: look at the stars at the same time of night on two dates a month apart and compare the patterns.

The Bhil and Pawara, Indigenous communities of the Tapi Valley in western India, used the appearance of certain star patterns as a sign that the monsoon rains were coming.