(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...

Answer Details

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

(i) Circumnavigation of the earth: Ships and aircraft travelling continuously in one direction can return to their starting point without turning back. This shows that the earth is rounded.

(ii) Sunrise and sunset: The sun appears gradually at sunrise and disappears gradually at sunset. This is because the earth has a curved surface and rotates on its axis.

(iii) Eclipse of the moon: During a lunar eclipse, the earth casts a circular shadow on the moon. Only a spherical body can always cast a circular shadow.

(b) Three effects of the revolution of the earth include:

(i) It causes seasons: As the earth revolves round the sun with its axis inclined at 66½°, different parts of the earth receive different amounts of solar heat at different times of the year. Thus, when it is summer in the Northern Hemisphere, it is winter in the Southern Hemisphere.

(ii) It varies the length of day and night: The length of day and night changes during the year. Around 21 June, the Northern Hemisphere experiences longer days and shorter nights, while the Southern Hemisphere experiences shorter days and longer nights. Around 22 December, the condition is reversed.

(iii) It changes the altitude of the midday sun: The vertical rays of the sun appear to migrate between the Tropic of Cancer and the Tropic of Capricorn. The sun is overhead at the Equator on about 21 March and 23 September, at the Tropic of Cancer on about 21 June, and at the Tropic of Capricorn on about 22 December.

(c) Calculation of the noon altitude of the sun at the Tropic of Capricorn on 21 June:

On 21 June, the sun is vertically overhead at the Tropic of Cancer, 23½°N.

The Tropic of Capricorn is at 23½°S.

The angular distance between the Tropic of Capricorn and the overhead sun is:

23½° + 23½° = 47°

Noon altitude of the sun = 90° − angular distance.

Therefore:

90° − 47° = 43°

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