(Justin Grieser)

Winter weather hit many parts of the U.S. early this year. Yet officially (using the astronomical and traditional definition), the new season begins with the winter solstice at 6:03 p.m. Eastern Time today.

Why are the shortest days not the coldest? And why was our earliest sunset already two weeks ago? Here are five questions (and answers) that explain the winter solstice…

1. Why are the days so short?


On the December solstice, the Northern Hemisphere is at its maximum tilt away from the sun and experiences significant amounts of darkness. (timeanddate.com)

On the December solstice, the sun appears directly overhead at 23.5 degrees south latitude, along the Tropic of Capricorn. While the Southern Hemisphere enjoys its longest day of the year, we here in the Northern Hemisphere see the sun follow its lowest and shortest path across the southern sky. For the next six months, the days will gradually lengthen as our hemisphere begins to tilt back toward the sun.

2. What time is sunrise and sunset on the winter solstice?

Winter solstice daylight in U.S. cities
Sunrise and sunset times in major U.S. cities on the December solstice. (Justin Grieser; data from timeanddate.com)

For the Northern Hemisphere, the winter solstice marks the shortest day and longest night of the year. The nation’s capital sees under 9 and a half hours of daylight on the solstice – but it could be worse. Northern cities like Seattle and Minneapolis get less than 9 hours of daylight, while the sun is up for barely 4 hours in Fairbanks, Alaska.

If the sun is shining on December 21 or 22, your shadow at local noon will be the longest of the year. The higher the latitude, the lower the sun appears in the sky, which means your shadow will be significantly longer in Seattle than in Atlanta or Miami.

3. Where does the sun rise and set along the horizon?


Position of sunrise and sunset in Washington, D.C. on the winter solstice. (suncalc.net)

Thinking about photographing the solstice sunset? Look to the southwest! On the December solstice, all locations on Earth (outside of the polar regions) see the sun rise and set at its southernmost point along the horizon.

The map of Washington, D.C. (above) shows sunrise and sunset occur within 120 degrees from due north along the horizon – well to the southeast and southwest. The higher your latitude, the closer sunrise and sunset appear to due south on a compass. Around the Arctic Circle, the location of sunrise and sunset converges in the southern sky, until eventually the sun never makes it above the horizon.

4. Why are the earliest sunset and latest sunrise not on the solstice?

Date and time of the earliest sunset and latest sunrise in major U.S. cities. (Justin Grieser)
Date and time of the earliest sunset and latest sunrise in major U.S. cities. (Justin Grieser)

The winter solstice marks the shortest daylight period, but it’s not the day of the latest sunrise or earliest sunset. In the mid-latitudes the earliest sunset occurs in early December, while the latest sunrise is not until early January. This misalignment occurs because of a discrepancy between “clock time” (which is based on 24 hours), and “solar time” (the time it takes for the sun to appear in the same position in the sky from one day to the next).

The Earth’s axis is tilted 23.5 degrees, and our orbit around the sun is elliptical (non-circular). In December, these two factors combine in such a way that our days are actually a few seconds longer than 24 hours – as seen by the amount of time it takes the sun to cross our local meridian (longitude) from one day to the next. In effect, this pushes the time of solar noon several minutes later during December, advancing both sunrise and sunset times even as the days continue to shorten until December 21.

Above we see that Washington, D.C. had its earliest sunset on December 8, while the latest sunrise is not until January 5. At higher latitudes, the date of the earliest sunset and latest sunrise occurs closer to the solstice, while closer to the equator (e.g. Miami), they occur more than a month apart.

5. Why do the coldest days of the year come after the solstice?


The coldest day of the year on average across the U.S., based on 1981-2010 climate normals. (NOAA/NCDC)

Even though daylight slowly increases after the solstice, many places don’t see their coldest days until mid-January. This happens because the Northern Hemisphere continues to lose more heat than it gains for several more weeks. The oceans – which take longer than land to heat up and cool down – play a role in this seasonal temperature lag. Only after the Northern Hemisphere starts to receive more solar energy than it loses do average temperatures begin their upward ascent.

Recently, NOAA’s National Climatic Data Center published a map showing when the U.S. sees its coldest temperatures of the year, based on 30-year climate averages. Above, we see that the intermountain West tends to see its coldest low temperatures closer to the winter solstice, while the coldest days in the Northeast usually don’t arrive until late January. The “average” coldest day of the year depends on several factors, including proximity to water and the timing and amount of snow cover.

Whether you love or hate winter, the winter solstice offers a ray of light for everyone: Snow lovers can rejoice that the coldest days are still upon us, while for warm weather fans, the lengthening days mean spring is just a few more months away.

Read more:

Winter Solstice 2013
Winter Solstice 2012
Winter Solstice 2011