Shape and Size

1.1 - Know that the shape of the Earth is an oblate spheroid

1.2 - Be able to use information about the mean diameter of the Earth (13,000 km)

Earth is not a perfect sphere but an oblate spheroid which means it's flattened at the poles and bulges at the equator. It is thought the rotation of Earth caused this. The diagram on the right is an exaggeration - in reality the difference is very small.

The Earth is 12,756km at the equator and 12,714km from pole to pole. We round this up to 13,000km.

There is evidence that early astronomers knew that Earth was round for a number of reasons:

  • The Moon is round.
  • The shadow of the Earth on the Moon during a lunar eclipse is round.
  • Travellers going north or south see different stars not visible from elsewhere.
  • Travellers recording shadows at different angles on the same date.
  • Tall ships appearing to 'sink' as they move over the horizon

 

Data
km > Radius Diameter Circ*
Equator 6,378.1 12,756.3 40,075
Poles 6,356.8 12,715.4 40,008
Difference 21.3 40.9 67

*Circ=Circumference

Summary

Earth's diameter is approximately 13,000km wide

Links

The Shape of the Earth Mathematical Imagery