It's not entirely accurate to say that the vertical rays of the sun only fall on the tropics. The vertical rays of the sun, also known as the solar declination, actually move between the Tropic of Cancer and the Tropic of Capricorn throughout the year.

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 direct sunlight throughout the year.

* Solar Declination: The angle of the sun's rays hitting the Earth changes throughout the year. This change is known as the solar declination.

* Movement of the Vertical Rays: Due to the Earth's tilt and its orbit around the sun, the vertical rays of the sun move between the Tropic of Cancer (23.5 degrees North) and the Tropic of Capricorn (23.5 degrees South).

* Summer Solstice: On the summer solstice (around June 21st), the vertical rays of the sun fall on the Tropic of Cancer. This is why the Northern Hemisphere experiences its longest day of the year.

* Winter Solstice: On the winter solstice (around December 21st), the vertical rays of the sun fall on the Tropic of Capricorn. This is why the Southern Hemisphere experiences its longest day of the year.

* Equinoxes: During the spring (around March 20th) and autumn (around September 22nd) equinoxes, the vertical rays of the sun fall on the equator.

Therefore, the vertical rays of the sun do not stay confined to the tropics; they move back and forth between the tropics over the course of a year, influencing the seasons and day lengths in different parts of the world.