(a) What is the shape of the earth? (b) With the aid of appropriate diagrams, give four proofs to support your answer in (a) above

(a) Shape of the earth

The earth is spherical (round) in shape, but not a perfect sphere. It is slightly flattened at the two poles and bulges at the equator; this true shape is described as an oblate spheroid (geoid).

(b) Four proofs that the earth is spherical

• Circumnavigation: Travellers and ships (from Magellan's voyage onward) who set out in one direction and keep going eventually return to their starting point without turning back, which is only possible on a round body. (Diagram: an arrow going right round a circle back to the start.)
• Ship on the horizon: As a ship approaches from over the sea, the top of its mast is seen first and then the hull; as it sails away the hull disappears before the mast. This is caused by the curvature of the earth's surface. (Diagram: ships at different points on a curved sea surface.)
• Circular shadow during a lunar eclipse: The shadow the earth casts on the moon during a lunar eclipse is always circular, and only a sphere casts a circular shadow in every position. (Diagram: sun, round earth and its round shadow falling on the moon.)
• The widening horizon / view from a height: The higher an observer climbs (a hill, an aircraft), the wider the circular horizon they can see, which shows the surface is curved. Photographs taken from space and aircraft also show the earth as a sphere. (Diagram: an observer at increasing heights with an expanding circular horizon.)