The anticipation of longer days is a nearly universal feeling after late autumn darkness sets in. The turning point for the Northern Hemisphere occurs on the date of the Winter Solstice. This astronomical event marks the shortest day and the longest night of the year. The next day begins the slow, steady progress toward more sunlight hours.
The Astronomical Turning Point
The Winter Solstice is the singular moment when the Northern Hemisphere reaches its maximum tilt away from the Sun. This astronomical event typically occurs on December 21st or 22nd. The Solstice signifies the lowest angle the Sun will reach in the sky at noon for the entire year.
This low arc across the horizon results in the fewest hours of daylight for any location north of the equator. The Solstice is not the midpoint of winter, but rather the official start of astronomical winter. For those in the Southern Hemisphere, the December Solstice marks their Summer Solstice and the longest day of their year.
After the Winter Solstice, the Sun’s path begins to shift slightly higher in the sky each day. This gradual change means that every subsequent day gains a few more seconds or minutes of sunlight. The increase is minuscule at first, becoming more noticeable in the weeks following the turn.
The Science Behind Changing Day Length
The seasonal change in day length is caused by the 23.5-degree axial tilt of the Earth. Our planet does not stand straight up and down as it orbits the Sun; instead, it is permanently tipped over on its axis. This tilt is the primary driver of the seasons and the varying duration of daylight hours across the year.
As the Earth travels around the Sun, the Northern Hemisphere is tilted away from the Sun during winter. This orientation results in the Sun’s rays striking the Northern Hemisphere at a more oblique, or slanted, angle. This slant causes the solar energy to be spread out over a larger surface area, which contributes to cooler temperatures and shorter days.
The Solstice occurs when the Northern Hemisphere’s tilt away from the Sun is at its maximum. At this point, the Sun’s direct rays are vertically overhead at the Tropic of Capricorn. As the Earth continues its orbit, the Northern Hemisphere begins to tilt less away from the Sun, causing the sun angle to increase and the days to lengthen.
The length of the day is determined by how long the Sun is visible above the horizon, which is directly affected by this axial tilt. The tilt dictates the duration of the Sun’s path across the sky for any given latitude. The change in day length is most dramatic at the poles and least noticeable near the equator, which always receives a consistent 12 hours of daylight.
Understanding the Sunrise and Sunset Delay
While the Winter Solstice officially has the shortest amount of total daylight, the dates of the earliest sunset and the latest sunrise do not occur on the same day. For mid-northern latitudes, the earliest sunset typically happens a couple of weeks before the Solstice, often around December 7th or 8th. Conversely, the latest sunrise does not take place until about two weeks after the Solstice, usually in early January.
This apparent discrepancy is due to the Equation of Time. Our modern clocks keep a steady 24-hour day based on the average length of a solar day, called mean solar time. However, the actual solar day, measured from one solar noon to the next, is not exactly 24 hours long and varies slightly.
The variation in the length of the true solar day is caused by two factors: the Earth’s elliptical orbit and the tilt of the Earth’s axis. Because the Earth speeds up when it is closer to the Sun and slows down when it is farther away, the time of solar noon shifts relative to clock time. This variable speed causes the sun to appear to run fast or slow compared to our clocks.
The combination of the Earth’s tilt and its orbital speed causes the time of solar noon to occur later each day around the winter period. Even though the Solstice adds daylight to the afternoon, the fact that solar noon itself is getting later pushes both the sunrise and sunset times later on the clock. This effect is strong enough to cause the earliest sunset to occur before the Solstice, and the latest sunrise to lag until the effect is overcome in January.