As the year approaches winter, many people notice the evenings getting darker earlier, a sure sign that daylight hours are shrinking. The common assumption is that the sun begins setting later only after the shortest day of the year has passed. However, the earliest sunset is a subtle astronomical phenomenon that occurs well before the shortest day, driven by the complexities of Earth’s orbital motion.
The Earliest Sunset Versus the Shortest Day
The winter solstice, which typically falls around December 21st in the Northern Hemisphere, marks the shortest day of the year, meaning it has the least amount of time between sunrise and sunset. Surprisingly, the earliest sunset does not occur on this day, but rather several weeks prior. For observers at mid-latitudes, such as 40 degrees North, the sun reaches its earliest setting time around December 7th or 8th.
This apparent discrepancy happens because the length of a solar day—the time it takes for the sun to return to its highest point (solar noon)—is not fixed at exactly 24 hours. Near the winter solstice, the solar day is slightly longer than the 24-hour day kept by our clocks. Consequently, the moment the sun reaches its peak altitude shifts later by a few seconds each day during this period.
The later-shifting solar noon gradually pushes both the sunrise and sunset times later on the clock. Although the total amount of daylight is still decreasing leading up to the solstice, the later solar noon causes the sun to begin setting a little later each afternoon starting in early December. This delay continues to affect the sunrise time until early January. Consequently, the earliest sunset precedes the shortest day, and the latest sunrise occurs weeks after it, creating an asymmetry in the morning and evening light shifts.
Understanding the Equation of Time
The reason the solar day length varies from our 24-hour clock is explained by the Equation of Time, a concept in celestial mechanics. This value describes the difference between apparent solar time (measured by a sundial) and mean solar time (the uniform time kept by clocks). This difference is caused by the combined influence of two distinct astronomical factors related to Earth’s motion in space.
Orbital Eccentricity
The first factor is the elliptical shape of Earth’s orbit around the sun. The distance between the Earth and the sun changes throughout the year. As Earth moves, it obeys Kepler’s second law of planetary motion, traveling faster when it is closer to the sun (perihelion, which occurs in early January).
When Earth speeds up in its orbit, the sun appears to move faster across the sky, making the time between successive solar noons longer than 24 hours. Conversely, when Earth is farthest from the sun (aphelion in early July), it moves slower, and the solar day is slightly shorter than 24 hours. This variable orbital speed contributes significantly to the daily shift in solar noon.
Axial Tilt
The second factor contributing to the Equation of Time is the 23.5-degree tilt of Earth’s rotational axis, known as the obliquity of the ecliptic. Even if the orbit were perfectly circular, the axial tilt would still cause the sun’s apparent speed along the celestial equator to change. At the solstices, the sun’s apparent path is most oblique to the equator, which contributes to lengthening the solar day.
The combination of the elliptical orbit and the axial tilt creates a predictable pattern of variation in the Equation of Time. This pattern can cause the sun to be up to 16 minutes ahead or 14 minutes behind clock time at different points in the year. The visual representation of this combined effect is the analemma, the figure-eight curve the sun traces in the sky if photographed at the same clock time every day for a year.
The Role of Daylight Saving Time
While astronomical factors cause the subtle, gradual shift in sunset times, the most dramatic change many people experience is the implementation of Daylight Saving Time (DST). DST is the practice of advancing clocks by one hour during the spring and summer months. This manipulation of civil time does not alter the sun’s actual position or the total duration of daylight, but it instantly changes the clock time of sunset.
When clocks “spring forward,” the time of sunset instantly appears one hour later on the clock, creating a sudden and noticeable lengthening of evening daylight. This abrupt change overshadows the gradual astronomical shift that occurs after the winter solstice. The main purpose of this shift is to better align human waking hours with the available daylight, especially during the summer.
Conversely, when DST ends in the autumn, the sunset time instantly appears one hour earlier on the clock. This “fall back” contributes significantly to the feeling of sudden darkness in the early evening. The effect of DST is a calendar-based adjustment that manipulates our perception of sunset time, separate from the celestial mechanics that govern the sun’s true movement.