The specific clock time of sunset on any given day, including the winter solstice, depends entirely on your geographic location, specifically your latitude and longitude. An observer in London will experience a different sunset time than someone in Cairo or New York City, even though they share the same astronomical event. To understand the solstice, it is more useful to examine the underlying celestial movements that govern our timekeeping.
Defining the Winter Solstice
The winter solstice is an astronomical event that marks the shortest day and longest night of the year for one of Earth’s hemispheres. For the Northern Hemisphere, this event typically falls on December 21st or 22nd. Astronomically, the solstice is the precise moment when one of Earth’s poles reaches its maximum tilt away from the Sun. This maximum tilt causes the Sun to travel its lowest and shortest arc across the sky, resulting in the minimum amount of daylight hours. Although the solstice is the day with the least overall daylight, it is a common misconception that this date also features the earliest sunset.
Why the Earliest Sunset Happens Before the Solstice
For most locations in the Northern Hemisphere, the earliest sunset of the year occurs in early December, often around December 8th, not on the solstice itself. This phenomenon is due to a discrepancy between the time kept by our standardized clocks and the time measured by the Sun’s actual position in the sky, known as solar time. Our clocks assume a uniform 24-hour day, but the time it takes for the Sun to return to its highest point (solar noon) varies slightly throughout the year.
Around the winter period, the length of the solar day is slightly longer than 24 hours. Because of this lengthening, the time of solar noon—the midpoint of the daylight period—creeps later each day. This shift causes both the sunrise and the sunset to occur later according to the clock.
The earliest sunset happens when the geometric effect of the shortening daylight hours is counteracted by the solar noon shift. This asymmetrical shift means that the earliest sunset has already passed, even as the total duration of daylight continues to shrink toward its minimum on the solstice.
Understanding the Equation of Time
The scientific explanation for why the solar day varies in length involves a concept called the “Equation of Time.” This equation quantifies the difference between the time indicated by a sundial (apparent solar time) and the time indicated by a clock (mean solar time). This difference can be as much as 16 minutes throughout the year, meaning solar noon does not always fall exactly at 12:00 p.m.
Two primary astronomical factors contribute to this equation. The first is the Earth’s elliptical orbit, which causes the planet to speed up as it gets closer to the Sun (perihelion) and slow down as it moves farther away. Since Earth is closest to the Sun in early January, its orbital speed is faster around the time of the solstice, making the apparent motion of the Sun irregular.
The second factor is the tilt of the Earth’s axis, known as the obliquity of the ecliptic, which is approximately 23.4 degrees. Because the Sun’s apparent path is tilted relative to the equator, the Sun’s daily eastward movement would appear non-uniform. The combination of these two effects causes solar noon to drift later in December and early January, which in turn pushes the timing of both sunrise and sunset later on our clocks.
Tracking Daylight Hours
Even though the sunset time begins to get later shortly after the earliest sunset date in early December, the total amount of daylight continues to decrease until the solstice. This is because the sunrise time is also creeping later each morning, and the decrease in morning light outweighs the small gain in evening light. Sunrise times do not reach their latest point of the year until well after the solstice, often around the first week of January.
The winter solstice is the true turning point for daylight, marking the day when the Northern Hemisphere starts to gain more total time between sunrise and sunset. After this date, the gradual shift in solar time combines with the seasonal change to produce both later sunsets and, eventually, earlier sunrises.