Winter may just be getting started, but if you’re ready for more sunlight, you won’t have much longer to wait. Dec. 21 is the winter solstice: the shortest day and longest night of the year here in Earth’s northern hemisphere.
Starting Friday, the sun will be up for a few seconds longer each day, signaling the start of our slow but steady march toward spring. But let’s not get ahead of ourselves. Winter is just getting into high gear. Learn more about the solstice and why it’s not the coldest day of the year in our scientific guide below.
1. What happens on the winter solstice?
The December solstice marks the exact moment when the sun’s most direct rays reach their southernmost pointsouth of the equator, along the Tropic of Capricorn, at 23.5 degrees south latitude. The time and date of the solstice change slightly each year, but this year’s solstice occurs at 11:28 a.m. Eastern Time on Dec. 21.
On the winter solstice, Earth’s northern hemisphere reaches its maximum tilt away from the sun. (NASA)
The reason we have a solstice — and seasons — is because the Earth is tilted on its axis of rotation by about 23.5 degrees. This tilt causes each hemisphere to receive different amounts of sunlight throughout the year as our planet orbits the sun. In the Northern Hemisphere, we see the sun take its lowest and shortest path across the southern sky, and at local noon, your shadow will be the longest of the year.
The word “solstice” comes from the Latin words sol sistere, which means “sun standing still.” On the December solstice, the sun’s daily southward movement in the sky appears to pause, and we see the sun rise and set at its southernmost points on the horizon before reversing direction. It’s a yearly astronomical turning point that humans have celebrated for millennia (just think Stonehenge or the ancient Maya).
2. How many hours of daylight are there on the winter solstice?
The amount of daylight you’ll see on the solstice depends on your latitude, or distance from the equator. The map below, created by Alaska-based climatologist Brian Brettschneider, shows how widely daylight hours vary across North America on the shortest day of the year.
In the Lower 48, the sun is up for more than 10 hours across Florida and southern Texas, while states across the northern tier get under nine hours of daylight. Here in Washington, D.C., the sun is up for 9 hours 26 minutes (rising at 7:23 a.m. and setting at 4:49 p.m.).
Of course, our long winter night pales in comparison with Alaska, where the sun barely climbs above the horizon for three to four hours in much of the Last Frontier. North of the Arctic Circle — at 66.5 degrees north latitude — the sun never rises, and darkness prevails as the Earth rotates on its axis.
3. When are sunrise and sunset?
The exact times of sunrise and sunset depend on two things: your latitude and geographic location within your time zone.
Here are two maps that show the time of sunrise and sunset across North America. Both take into account the effect of time zones and latitude, hence the interesting patchwork of colors.
The first map shows sunrise times across North America. In most of the country, including the District, sunrise on the winter solstice is after 7 a.m. Golden-colored areas don’t see sunrise until after 7:30 a.m., and in green areas, the sun doesn’t rise until after 8 a.m.
The next map shows the time of sunset. Many parts of the Lower 48 see sunset before 5 p.m. on the winter solstice. In bright green areas, including much of the Pacific Northwest and New England, the sun disappears below the horizon before 4:30 in the afternoon. Parts of Maine even see sunset in the 3 o’clock hour! Only a handful of states, including Florida and Texas, see sunset after 5:30 p.m. on the shortest day of the year.
If you’re tired of these dark evenings, the good news is that our earliest sunsets are already behind us. In fact, it’s been gradually getting lighter in the evenings for more than a week now.
4. Wait a minute, the earliest sunset and latest sunrise don’t occur on the solstice?
Let’s clear the record: The winter solstice marks the shortest daylight period in the Northern Hemisphere. However, it’s never the day of the latest sunrise or earliest sunset. This astronomical quirk happens because of Earth’s 23.5-degree tilt and our elliptical orbit around the sun (read more).
You can see in this next map (and in this table) that most places see their earliest sunset two weeks before the solstice, while the latest sunrise isn’t until early January. So don’t expect brighter mornings anytime soon.
Calculated down to the second, the District’s earliest sunset (4:46 p.m.) was on Dec. 7. Meanwhile, the latest sunrise (7:27 a.m.) isn’t until Jan. 5. The closer you move to the North Pole, the closer the earliest sunset and latest sunrise occur to the solstice.
5. Why do the days still get colder after the solstice?
“As the days lengthen, the cold strengthens.” It’s an old proverb that certainly has some scientific truth. The Northern Hemisphere receives its least direct sunlight on the winter solstice, but in many places the coldest average temperatures of winter aren’t until January, as shown in this final map:
This delay in the arrival of our coldest temperatures is better known as seasonal lag. It happens because the amount of solar energy arriving at the ground is less than the amount leaving the earth for a few more weeks (a bit like a bank account that starts losing money when you make more withdrawals than deposits). Oceans and bodies of water — which take longer than land to heat up and cool down — keep temperatures from rising very fast. Not until the Northern Hemisphere sees a net gain in solar energy (more heat coming in than going out) do average temperatures begin their ascent.
The exact timing of the coldest stretch of the year depends on several factors, including how close you live to water, prevailing wind direction and the amount of snow cover (snow is great at reflecting the sun’s heat straight back into space). You’ll notice in the map above that Western states typically see their coldest stretch of winter closer to the solstice, while areas near the Great Lakes and interior New England don’t see their coldest days until late January.
If you don’t like the cold, here’s a silver lining: Whatever the rest of winter brings, daylight is once again on the upswing. That’s definitely something to celebrate!
Reference:https://www.washingtonpost.com/news/capital-weather-gang/wp/2017/12/21/winter-solstice-2017-five-things-you-should-know-about-the-shortest-day-of-the-year/?utm_term=.cb7b2e2eeece
Reference:https://www.washingtonpost.com/news/capital-weather-gang/wp/2017/12/21/winter-solstice-2017-five-things-you-should-know-about-the-shortest-day-of-the-year/?utm_term=.cb7b2e2eeece
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