The Winter Solstice marks the day with the shortest period of daylight in the Northern Hemisphere. The precise time of sunset is not a fixed moment but changes dramatically based on location. This time is determined by a combination of astronomical and geographical factors, explaining why two cities can experience the shortest day at different clock times. This variability requires looking at the Earth’s orbit, its tilt, and time zones.
Defining the Winter Solstice
The Winter Solstice is an astronomical event that occurs when one of the Earth’s poles reaches its maximum tilt away from the Sun. This tilting is caused by Earth’s axial tilt of approximately 23.5 degrees relative to its orbital plane. For the Northern Hemisphere, this event occurs around December 21st or 22nd each year. The maximum tilt away from the Sun causes the solar altitude, or the Sun’s highest point in the sky, to be at its lowest arc of the entire year. As a result, the day of the Winter Solstice experiences the shortest duration of daylight and the longest night.
The solstice itself is defined as the precise moment the Sun reaches its southernmost declination, a celestial coordinate similar to latitude. This point in the Earth’s orbit signals the reversal of the Sun’s apparent ebbing in the sky, meaning that immediately after the solstice, the daylight hours begin to increase again.
The Role of Latitude and Longitude in Sunset Time
The precise clock time of sunset on the solstice is primarily determined by two geographical coordinates: latitude and longitude. Latitude measures distance north or south of the equator and directly determines the total length of daylight. The further north a location is, the shorter its daylight period will be on the solstice, resulting in an earlier sunset compared to locations closer to the equator.
Longitude, which measures distance east or west, influences the clock time of sunset because of time zones. All locations within a time zone share the same clock time, even though the sun’s position relative to them is different. Cities situated on the western edge of a time zone will experience sunset later by the clock than those on the eastern edge of the same time zone.
Why the Earliest Sunset Isn’t on the Solstice
A common misconception is that the earliest sunset of the year occurs on the Winter Solstice, the day with the least daylight, but this is not the case. The earliest sunset actually occurs weeks before the solstice, typically in early to mid-December. This phenomenon is explained by the difference between “clock time” and “solar time,” which is mathematically defined by the Equation of Time.
The Equation of Time
The Equation of Time describes the variation between the time shown on a clock (mean solar time) and the time indicated by a sundial (apparent solar time). This variation is caused by two astronomical factors: the Earth’s axial tilt and its elliptical orbit around the Sun. The orbit is not a perfect circle, meaning the Earth speeds up when it is closer to the Sun.
Near the Winter Solstice, the effect of Earth’s elliptical orbit causes the true solar day—the time from one solar noon to the next—to be slightly longer than the 24-hour clock day. This continuous lengthening causes solar noon to occur progressively later each day. Because solar noon shifts later, both sunrise and sunset are delayed, pushing the time of the latest sunset to occur after the shortest day. The effect of the Equation of Time is consistent across all latitudes.
Tracking Daylight Change After the Solstice
After the Winter Solstice passes, the length of daylight begins to increase, though the change is initially very gradual. For locations in the mid-latitudes, the gain in daylight is often only a few seconds per day immediately following the solstice. The rate of change accelerates as the Earth moves closer to the spring equinox, reaching two minutes of daylight per day by mid-January.
This post-solstice period is characterized by later sunsets, reinforcing the point that the earliest sunset occurred weeks prior. The latest sunrise of the year also occurs after the solstice, typically in early January. The continued shift in solar noon, driven by the Equation of Time, pushes the time of sunrise later, making the mornings feel darker even as the afternoons slowly begin to brighten.