The sun is vertically overhead at the tropic of Capricorn on

Analyze the question

The question asks for the date on which the sun is directly overhead (at 90° to the earth's surface) at the Tropic of Capricorn, which lies at 23½°S.

Explain the concept

The sun appears to move between the Tropic of Cancer (23½°N) and the Tropic of Capricorn (23½°S) during the year. This happens because the earth's axis is tilted at 23½° while it revolves around the sun. The key dates are:

| Date | Event | Overhead sun at | |---|---|---| | March 21st | Spring (vernal) equinox | Equator (0°) | | June 21st | June solstice | Tropic of Cancer (23½°N) | | September 23rd | Autumn equinox | Equator (0°) | | December 22nd | December solstice | Tropic of Capricorn (23½°S) |

Step-by-step solution

1. The Tropic of Capricorn is in the Southern Hemisphere (23½°S).
2. The sun reaches its southernmost position during the Southern Hemisphere's summer, around December.
3. A. June 21st is when the sun is overhead at the Tropic of Cancer, so it is not correct.
4. B. September 23rd and C. March 21st are equinoxes, when the sun is overhead at the Equator, so they are not correct.
5. D. December 22nd is the December solstice, when the sun is overhead at the Tropic of Capricorn, so it is correct.

Answer: D. December 22nd

Exam tip

Remember: "June goes North, December goes South." The equinoxes (March and September) are always the Equator. Also, when the sun is overhead at the Tropic of Capricorn, it is summer in the Southern Hemisphere and winter in the Northern Hemisphere (so Nigeria experiences its harmattan period around then).