Final answer:
The vertical rays of the Sun strike the Earth at the Tropic of Cancer during the Northern Hemisphere's summer solstice, around June 21st, when the Sun is at a declination of 23° N.
Step-by-step explanation:
The vertical rays of the Sun strike the Earth at the Tropic of Cancer on the summer solstice, which occurs on or about June 21st. On this day, the Sun's declination is at 23° N of the celestial equator, aligning with the Tropic of Cancer's latitude, allowing the Sun to be directly overhead at noon in this region. The phenomenon marks the beginning of summer in the Northern Hemisphere. If the Earth's tilt were less than its current 23.5 degrees, the location of the Tropic of Cancer would shift closer to the equator.