The term “solstice” comes from the Latin words sol (sun) and sistere (to stand still), reflecting the moment when the Sun’s apparent movement north or south pauses before reversing direction. Solstices occur twice each year, marking the points of maximum and minimum daylight hours. These astronomical events are directly related to the length of the day, which is defined as the time between sunrise and sunset. The solstices establish the extreme limits of the annual cycle of daylight and darkness.
The Extremes of Daylight: Summer and Winter Solstices
The solstices define the two days of the year with the most and fewest hours of sunlight for any given hemisphere. The Summer Solstice, occurring around June 20 to 22 in the Northern Hemisphere, is the day with the greatest number of daylight hours. Conversely, the Winter Solstice, which takes place around December 20 to 23, receives the fewest hours of daylight and the longest period of night.
These extremes are precisely reversed for the Southern Hemisphere. When the Northern Hemisphere experiences its Summer Solstice in June, the Southern Hemisphere is simultaneously experiencing its Winter Solstice and its shortest day of the year. Similarly, the December solstice brings the longest day to the Southern Hemisphere and the shortest day to the North. The variation in day length becomes more pronounced the further a location is from the equator, with regions like the Arctic Circle experiencing 24 hours of continuous daylight during their summer solstice.
The Astronomical Cause: Earth’s Axial Tilt
The shifts in day length are not caused by the Earth’s changing distance from the Sun; the planet is actually closest to the Sun in January. Instead, the phenomenon is entirely driven by the Earth’s axial tilt, which is a constant 23.5-degree angle relative to its orbital plane. This tilt ensures that different hemispheres are angled either toward or away from the Sun as Earth revolves around it.
During the Summer Solstice, the tilt causes one hemisphere to be angled maximally toward the Sun. This results in the Sun’s most direct rays hitting the Tropic of Cancer, located at 23.5 degrees North latitude. The high angle of incidence leads to greater solar intensity and longer daylight hours in the hemisphere tilted toward the Sun.
At the Winter Solstice, the opposite occurs, with that hemisphere tilted maximally away from the Sun. The Sun’s most direct rays fall upon the Tropic of Capricorn at 23.5 degrees South latitude. The hemisphere experiencing winter receives the Sun’s energy at a low angle, forcing the light to travel through a greater thickness of atmosphere, which reduces the solar intensity and results in the shortest possible day.
The Immediate Shift in Day Length Trends
The solstice is a turning point, marking the moment when the trend in day length reverses. Following the Summer Solstice, the days immediately begin to shorten as the hemisphere starts to angle away from the Sun, continuing toward the Autumnal Equinox.
Conversely, the Winter Solstice is the day the shortening of daylight hours ceases. The amount of daylight begins to increase the very next day, starting the slow march toward the Vernal Equinox. This reversal, however, does not always align perfectly with the earliest sunset or latest sunrise times. This is due to the slight variation in the length of a solar day caused by Earth’s elliptical orbit, meaning the timing of sunrise and sunset can shift by a few days from the solstice date.