Winter solstice brings longest night of year, not earliest sunset

December 21, 2013 by  
Filed under Solar Energy Tips

This Saturday, the 2013 winter solstice will occur at 12:11 p.m. EST on December 21, when the sun appears directly overhead along the Tropic of Capricorn, at 23.5 degrees south latitude. For the northern hemisphere, the date also marks the shortest daylight period of the year and the longest night.

We experience seasons because the Earth is tilted on its axis relative to the plane of its annual revolution around the sun. That means at any given time of the year, one part of the planet is more directly exposed to the sun’s rays than the other, so that when it’s winter in the northern hemisphere, it’s summer in the southern hemisphere.

On the winter solstice, the Earth’s north pole is at its maximum tilt away from the sun and at locations north of the equator, the sun follows its lowest and shortest arc across the southern sky. For the next six months–from Dec. 21 until the summer solstice on June 21–days will steadily grow longer. Locations in the southern hemisphere, by contrast, see the sun follow its highest and longest arc across the northern sky.

In Washington D.C., the winter solstice sun rises and sets within 120 degrees from due north along the horizon. Compare this to 90 degrees from due north on the equinoxes, when the Earth’s equatorial plane passes the center of the sun and its axis is tilted neither towards nor away from the sun.

Even though the shortest day of the year falls on the winter solstice, it doesn’t follow that the earliest sunset also occurs on that day.

Because the Earth’s orbit is elliptical rather than circular, combined with our planet’s 23.5-degree axial tilt, the earliest sunsets in the middle latitudes occur more than a week earlier than the winter solstice. In late December, the Earth is nearly at perihelion–the point in its orbit when it’s closest to the sun–which causes Earth to travel faster around its star, so that the sun takes less than 24 hours to appear in the same spot in the sky as the day before.

On the winter solstice in the northern hemisphere, incoming solar energy is at its minimum. Nevertheless, the Earth as a whole receives almost 7 percent more solar radiation in December than in June because our planet is about 3.1 million miles closer to the sun.

While locations north of the equator start receiving more solar energy after the winter solstice, the northern hemisphere still continues to lose more heat than it gains for about a month. So, even though the days are lengthening, temperatures will tend to continue to drop through the middle of January.

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