(a) State three reasons to show that the earth is spherical (b) Describe three effects of the revolution of the earth (c) Calculate the noon altitude of the...

(a) Three reasons to show that the earth is spherical

1. Circumnavigation: ships and aircraft travelling steadily in one direction eventually return to their starting point without reversing, which is only possible on a rounded body.
2. Ship and horizon evidence: as a ship approaches, the top of its mast is seen first and the hull last (and the reverse as it leaves); the horizon always appears circular from any high point.
3. Earth's circular shadow and space photographs: during a lunar eclipse the earth casts a circular shadow on the moon, and photographs taken from space show the earth as a sphere. The disappearance of the lower part of distant objects and the changing altitude of the Pole Star with latitude also confirm it.

(b) Three effects of the revolution of the earth

1. The changing seasons (spring, summer, autumn, winter) result as the earth revolves round the sun with its tilted axis.
2. Varying length of day and night at different times of the year, especially away from the equator.
3. Changing altitude and apparent migration of the noon sun between the Tropics, and the occurrence of the equinoxes and solstices; it also fixes the length of the year (about 365¼ days), giving leap years.

(c) Noon altitude of the sun at the Tropic of Capricorn on June 21

On 21 June (the June solstice) the overhead (vertical) sun is at the Tropic of Cancer, 23½°N. The Tropic of Capricorn is 23½°S.

The angular distance between the place and the point of the overhead sun is:

23½° + 23½° = 47°.

Noon altitude of the sun = 90° minus this angular distance:

\[ 90^{\circ} - 47^{\circ} = 43^{\circ} \]

The noon altitude of the sun at the Tropic of Capricorn on 21 June is 43°.