June 21 was summer solstice, the longest day of the year — as compared to the night — in the northern hemisphere. Winter solstice occurs on December 21 or 22, when the night hours are the longest. But why are the hours of daylight not the same every day?
The explanation lies in Earth’s tilt. The planet’s axis of rotation is tilted at an angle of 23.5°C. This tilt — combined with factors such as Earth’s spin and orbit — leads to variations in the duration of sunlight on any location on different days of the year. The tilt is also responsible for the different seasons.
Day & night
Day occurs on the side facing the Sun, and changes to night as Earth continues to spin on its axis. On the Equator, day and night are equal. The closer one moves towards the poles, the more extreme the variation. During summer in either hemisphere, that pole is tilted towards the sun and the polar region receives 24 hours of daylight for months. During winter, the region is in total darkness for months.
A latitude is a measure of a location’s distance from the Equator. Earth’s tilt helps define some familiar imaginary lines, which are also key to determining when a solstice occurs. At latitudes of 23.5° (matching the tilt) are the Tropics of Cancer and Capricorn, north and south of the Equator. At 66.5° (or 90° minus 23.5°) are the Arctic and Antarctic Circles, north and south. It is at latitudes higher than 66.5° (in either direction) that days of constant darkness or light occur.
On each Tropic, the sun is directly overhead at noon once a year. When this happens on the Tropic of Cancer, it is summer solstice in the northern hemisphere. When on the Tropic of Capricorn, it is winter solstice.
On the Equator, the sun is directly overhead on two days. These are the spring equinox in March and the autumn equinox in August. Across Earth, day and night are of equal length on these two dates. On the Equator, day and night are equal every day.