Final answer:
The sun's direct rays are overhead at the equator during the equinoxes on March 21 and September 21, creating equal day and night lengths globally. During the solstices, around June 21 and December 21, the direct rays are overhead at the Tropic of Cancer and the Tropic of Capricorn, respectively, resulting in the longest days in the corresponding hemispheres.
Step-by-step explanation:
Throughout the year, due to the tilt of Earth's axis, the direct rays of the sun reach different latitudes. Around March 21 and September 21, the Earth experiences the vernal (spring) and autumnal (fall) equinoxes respectively, during which the sun is overhead at the celestial equator and day and night are approximately of equal length everywhere on Earth. These dates mark the only times when the sun's direct rays shine overhead along the equator.
Then come the solstices, which occur around June 21 and December 21. During the June solstice, the direct rays of the sun are overhead at the Tropic of Cancer (23.5° N latitude), marking the longest day in the Northern Hemisphere. Conversely, during the December solstice, the Tropic of Capricorn (23.5° S latitude) receives the sun's direct rays, resulting in the longest day in the Southern Hemisphere.
In regions like the United States, the sun's path across the sky varies significantly with the seasons, leading to substantial differences in daylight hours. For example, during the summer solstice, the Northern Hemisphere experiences long, warm days due to the high angle and long duration of sunlight, whereas the situation is reversed during the winter solstice with shorter, colder days.