Here's why:

* The Earth's Tilt: The Earth is tilted on its axis at approximately 23.5 degrees. This tilt is the primary reason for seasons.

* Summer Solstice: During the summer solstice in the Northern Hemisphere (around June 21st), the North Pole is tilted towards the Sun, with the Sun's rays directly hitting the Tropic of Cancer (23.5 degrees North).

* Winter Solstice: During the summer solstice in the Southern Hemisphere (around December 21st), the South Pole is tilted towards the Sun, with the Sun's rays directly hitting the Tropic of Capricorn (23.5 degrees South).

Therefore, the Sun's rays hit latitude 23.5 degrees at a 90-degree angle at the summer solstice in the respective hemisphere.