Here's why:

* The Tropic of Capricorn: Latitude 23.5 degrees S is known as the Tropic of Capricorn.

* Earth's Tilt: The Earth is tilted on its axis at approximately 23.5 degrees. This tilt causes different parts of the Earth to receive varying amounts of sunlight throughout the year.

* Solstices: The solstices mark the days when the sun reaches its highest or lowest point in the sky. The December solstice (around December 21st or 22nd) marks the day when the sun's rays are directly overhead at the Tropic of Capricorn. This means the angle of the sun's rays is 90 degrees at this latitude.

Important Note: It's crucial to remember that the sun's rays are only directly overhead at 23.5 degrees S on the December solstice. During other times of the year, the angle of the sun's rays will be different.