The winter solstice marks the shortest day of the year in the Northern Hemisphere, typically occurring around December 21st. This astronomical event signals a turning point, promising the slow but steady return of light. Immediately after the solstice, the duration of daylight begins to increase, leading to a gradual shift toward earlier sunrises and later sunsets. Understanding this change requires looking at the relationship between Earth and the sun.
The Astronomical Reason for Changing Daylight Hours
The change in daylight is a direct consequence of the Earth’s consistent axial tilt and its orbit around the sun. The planet is tilted on its axis by approximately 23.5 degrees, remaining fixed in space throughout the year. The winter solstice occurs when the Northern Hemisphere is tilted maximally away from the sun.
This maximum tilt results in the sun’s path across the sky having the lowest angle and shortest duration. Once the solstice passes, the Northern Hemisphere slowly begins to tilt back toward the sun. This movement causes the sun’s most direct rays to track progressively northward.
The sun is visible for a longer period each day because the angle of the sun’s rays and the arc it traces above the horizon slowly lengthens. This geometric shift represents the start of the six-month march toward the summer solstice.
Calculating the Rate of Daylight Gain
The daily gain in daylight immediately following the winter solstice is small, starting with only a few seconds per day in the mid-latitudes. This slow initial rate occurs because the sun’s most southerly position in the sky appears to pause, which is why “solstice” means “sun stands still.”
For locations around 40 degrees North latitude, such as New York or Chicago, the gain is minimal for the first week, often less than a minute of total daylight. The rate of increase then accelerates noticeably into January and February. By mid-January, these regions typically gain around two minutes of daylight each day.
The maximum rate of daily daylight gain occurs around the time of the spring equinox in March, not immediately after the solstice. At this point, the sun’s apparent northward movement is fastest, leading to the greatest daily change, sometimes reaching three minutes or more per day. Following the equinox, the rate of gain slows again until the summer solstice.
How Latitude Affects the Daily Increase
The amount of daylight gained after the solstice depends heavily on a location’s latitude, becoming more dramatic the farther a place is from the equator. At the equator, the length of the day remains relatively constant throughout the year. Residents experience very little change, maintaining a consistent twelve hours of day and twelve hours of night.
In contrast, locations at very high latitudes experience the most pronounced shift in daylight hours. Near the Arctic Circle, the transition from polar night—where the sun does not rise—to the return of light is rapid. Following the solstice, these northern regions quickly gain many minutes and sometimes hours of light per day, leading to a steep curve of daylight change.
Comparing two US states illustrates this difference: a person in Florida (low latitude) will see a minimal daily change after the solstice. A person in Alaska (high latitude) will experience a much faster increase in the duration of sunlight. The closer one is to the poles, the more extreme the annual swing between maximum and minimum daylight becomes.
The Asymmetry of Shortest Day and Earliest Sunset
A common observation is that the earliest sunset of the year does not happen on the winter solstice. The earliest sunset occurs about one to two weeks before the solstice due to a celestial phenomenon known as the Equation of Time.
The Equation of Time describes the difference between clock time (mean solar time) and time measured by the sun’s actual position (apparent solar time). This discrepancy arises because the Earth’s orbit is slightly elliptical, causing its orbital speed to vary throughout the year.
Around the winter solstice, the Earth moves faster in its orbit, making the duration from one solar noon to the next slightly longer than 24 hours. This slightly longer solar day causes both sunrise and sunset to shift later each day during early December.
While the total length of daylight shrinks until the solstice, the sunset time is already shifting later. Consequently, the latest sunrise of the year occurs about one to two weeks after the solstice, even though the overall amount of daylight has already begun to increase.