(a) Three reasons to show that the earth is spherical include
(i) The circumnavigation of the earth
(ii) Sunrise and sunset
(iii) Eclipse of the moon.
(b) Three effects of the revolution of the earth include the follow
(i) It varies the length of day and night.
(ii) It causes seasons
(iii) It changes the altitude of the midday sun at differ times of the year
(i) Varying the length of day and night and seasons:- The earth is inclined at an angle of 66½ °N. In the Northern hemisphere in winter, during the month of December as we go northwards , hours and darkness increase. At artic circle (66½ °N) the sun never rises and there is darkness for the whole day around December 22nd. In summer around June, the conditions are reversed. Daylight increases as we go poleward. At the artic circle the sun never set at mid-summer (June 21st) and there is a complete 24 hour period of continuous daylight in the Southern hemisphere: the same process takes place except that the conditions are reversed. When it is summer in the Northern hemisphere the Southern hemisphere will experience winter.
Changes in the altitude of the midday sun: The sun is overhead vertically at the equator for two days each year. The days are March 21st and September 23rd. These two days are called Equinoxes (meaning equal days any nights). Because of these two days, all parts of the world will experience equal days and equal nights. After the March equinox, the sun appears to move northward and vertically overhead at the Tropic of Cancer (23½°N) around June 21st. This is known as June or summer solstice (when the Northern Hemisphere will have its longest day shortest nights). By December 22nd, the sun will be overhead at the tropic or capricorn (23½ºS) called December or winter solstice, i.e when the Southern Hemisphere has its longest day and shortest night
(c) To calculate the noon altitude of the sun at the Tropic of capricorn on June 21. The sun is overhead at the Tropic of capricorn i.e, 23½°S. Angle at right angle = 90° since the angle at the Tropic of capricorn is 23½°S. The noon altitude on June 21 of the sun is 90° - 23½ºS = 66½