Winter solstice: Shortest day of the year, but sunset already creeping later

Winter may just be getting started, but those looking forward to a bit more daylight have not much longer to wait. This Saturday is the winter solstice in the Northern Hemisphere, marking our shortest daylight period and longest night of the year.

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At 12:11 p.m. EST on December 21, the sun appears directly overhead along the Tropic of Capricorn, at 23.5 degrees south latitude. With the Earth’s north pole at its maximum tilt away from the sun, locations north of the equator see the sun follow its lowest and shortest arc across the southern sky. For the next six months, the days again grow longer as the sun spends more time above the horizon.

CWG’s solstice posts from previous years offer a wealth of information about the astronomical significance of the solstice. Here is a recap, along with a few new factoids:

Shortest and lowest sun path

On the winter solstice, areas of the Northern Hemisphere see the shortest daylight period of the year and the midday sun is at its lowest point in the sky.

In Washington, D.C., the sun is above the horizon for only 9 hours and 26 minutes, climbing 27.7° above the horizon at solar noon (12:06 p.m.). At higher latitudes in the Northern Hemisphere, the days are even shorter and the sun appears closer to the horizon.

Southernmost sunrise and sunset

All locations on Earth see the sun rise and set at its southernmost point along the horizon on the December solstice. This also holds true in the Southern Hemisphere, where the sun takes its longest and highest path across the northern sky.

In D.C., the sun rises and sets within 120° from due north along the horizon (compared with 90° from due north on the equinoxes when the sun rises at due east and sets at due west on the horizon).

The higher the latitude, the closer sunrise and sunset appear to due south on a compass. As you keep moving toward the Arctic Circle, the location of sunrise and sunset will converge in the southern sky, until eventually the sun never makes it above the horizon.

Earliest sunset and latest sunrise not on the winter solstice

Due to the eccentricity (elliptical shape) of Earth’s orbit around the sun, combined with our planet’s 23.5-degree axial tilt, the earliest sunsets of the year occur nearly two weeks before the winter solstice in the mid-latitudes. The simple explanation is that in late December the Earth is nearly at perihelion. This shorter distance from the sun causes Earth to move faster in its orbit, which means it takes a few seconds more than 24 hours for the sun to reach the same point in the sky each day (see image).

These extra seconds add up to a noticeable shift in solar noon times throughout December, pushing the start and end times of daylight a few minutes later each week. Since the sun’s declination – or height with respect to the horizon – changes little near the solstice, the later time of solar noon is the dominant influence on when the sun rises and sets. The table above shows that D.C. has its earliest sunset around December 8, while the latest sunrise is not until early January.

The Atlantic recently published an excellent article with more information about this sun and clock time discrepancy, known as the equation of time.

Minimum solar energy in the Northern Hemisphere

On the December solstice incoming solar radiation is at a minimum in the Northern Hemisphere. Yet the Earth as a whole actually receives 6.9 percent more solar energy in late December than in June since we are about 3.1 million miles closer to the sun. Locally, of course, the difference in incoming solar energy depends on both your hemisphere and latitude. Since the Northern Hemisphere is tilted away from the sun this time of year, places north of the equator receive significantly less energy from our star. In D.C., for example, we receive about 3.5 times less insolation in December than during the month of June.

The higher the latitude, the greater the difference in insolation between the winter and summer solstices. Fairbanks, Alaska gets about 285 times less solar energy in December than it will six months from now. Note that these changes in insolation are based on long-term averages, and don’t account for local cloud cover (typically more common in winter). This means the difference in energy reaching the Earth’s surface between summer and winter is often greater than these numbers suggest.

While areas north of the equator start to gain energy from the sun after the winter solstice, the Northern Hemisphere continues to lose more heat than it gains for the next month. This is why even as the days grow longer, our average temperatures don’t reach their minimum until mid-January. If you don’t like the cold, the good news is whatever the rest of winter brings, daylight is once again on the upswing.