The noon sun is at zenith at the Tropic of Cancer (23.5° North) on the summer solstice (around June 21st) and at the Tropic of Capricorn (23.5° South) on the winter solstice (around December 21st).

Here's why:

* 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.

* Solar Declination: The Sun's apparent position in the sky changes throughout the year. This change is known as solar declination. The solar declination reaches its maximum at the Tropic of Cancer during the summer solstice and its minimum at the Tropic of Capricorn during the winter solstice.

Therefore, at the Tropic of Cancer and Capricorn, the Sun is directly overhead at noon on their respective solstices.