(CNN) Thursday is the winter solstice -- the shortest day of 2017.
December 21 also marks the first day of astronomical winter -- although meteorological winter began December 1. In the Northern Hemisphere, where most of the world's population lives, the winter solstice marks the shortest day of the year and the longest night.
The good news for sunlight seeking Northern Hemisphere natives is that the days start getting longer beginning Friday -- and they can start counting down to spring.
It's the shortest day of the year because, during the winter solstice, the Northern Hemisphere is tilted 23.5 degrees away from the sun and the sun's position is at its most southerly point, directly over the Tropic of Capricorn -- right across the middle of Australia.
Solstice celebrations
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View Images Revellers gather at Stonehenge to see the sun rise as part of a winter solstice ceremony in 2015.
Photograph by Matt Cardy, Getty Images
Shortening days and falling temperatures are heralding the arrival of winter across the Northern Hemisphere.
This year, the northern winter solstice falls on December 21 at 11:28 a.m. ET. The solstice happens at the same moment around the world, though its local time depends on which of the 24 times zones you call home. And south of the Equator, the December solstice marks the onset of summer.
What is a Solstice? What is the definition of a solstice, and when do they happen? Is the winter solstice always the first day of winter? Find out what causes these bi-annual astronomical events, as well as how they bring about the shortest and longest days of the year.
For people in the north, is the year's shortest day also the official beginning of winter? Why does the solstice occur anyway, and how have people observed it over the ages? Read on for everything you need to know about the December solstice.
When Is the First Day of Winter?
The reason for the solstice—and the seasons—is that Earth is tilted with respect to the sun at an average of 23.5 degrees. This means the Northern and Southern Hemispheres receive unequal amounts of sunlight over a year as we orbit our star.
Each hemisphere cools down during the part of the year it's tilted away from the sun. The winter solstice (December in the north, June in the south) arrives at the time when that tilt is at its most extreme angle.
This astronomical event appears on calendars as the first day of winter—but meteorologists have already gotten a head start on the season. By the time the solstice rolls around, climate scientists have been observing winter for nearly a month, says Greg Hammer of NOAA's National Centers for Environmental Information.
“Meteorological winter in the Northern Hemisphere is always the months of December, January, and February, because those tend to be the coldest months of the year,” he says. “It's entirely based on the annual temperature cycle, rather than astronomically based.”
Considering sunlight's enormous influence on Earth's climate, why isn't the darkest part of the year also the coldest?
“Basically, it takes a while for water and landmasses to cool down from all the heat they have absorbed” during summer, Hammer says. “So, the minimum of daylight isn't followed by the coldest temperatures until about a month later. It varies from place to place, but around the third week in January tends to have the coldest temperatures of the year for most of the U.S.”
Meteorological winter more closely mirrors our civic calendars and the way most people actually think about the seasons, Hammer adds. “We think of winter being the coldest time and summer being the warmest time, with spring and fall being transitions.”
And for climate scientists, following meteorological winter offers important simplicity: “It just makes it easier in terms of calculating statistics and making comparisons from one season to another,” Hammer says. “The timing of the solstice can vary a bit. This way, we are comparing apples to apples when speaking seasonally, like noting that a certain winter was the coldest on record in a certain area.”
Earliest Sunset? Not on the Solstice.
Most of us see the year's earliest sunset a week or two before the solstice. That’s because the sun and our human clocks don't keep exactly the same time.
We've organized our days into 24-hour segments, but Earth doesn't spin on its axis that precisely. While the time from noon to noon is always exactly 24 hours, the time varies between solar noons, the moment each day when the sun reaches its highest point in the sky. As we move through the year, the chronological time of solar noon shifts seasonally—and so do each day's sunrises and sunsets.
During December, solar noons can happen some 30 seconds after we would mark a 24-hour cycle. While the shortest amount of total daylight falls on the solstice, that day's sunset is actually a few minutes later on our clocks than it was earlier in the month.
In Washington, D.C., for example, sunset on the solstice will be at 4:49 p.m. ET—four minutes later than it was back on December 9.
Closer to the Equator, the year's earliest sunset occurs in November. To see it coincide more closely with the solstice, simply head toward the Arctic. Seasonal shifts in the sun's path across the sky are most extreme closer to the Poles, which makes sunset times at high latitudes match the solstice schedule more closely.
Can I See the Solstice?
It is possible to see the effects of the solstice by noting what happens in the skies overhead, as well as the changes in sunlight over time.
For viewers in the north, the sun's arc across the sky has been steadily dropping lower and becoming shorter since June. At the northern winter solstice, it reaches its lowest possible arc—so low that in the few days surrounding the solstice, it appears to rise and set in the same place. That phenomenon produced the Latin origins of the word “solstice,” which means “sun stands still.”
The sun's low angle means that your noontime shadow is the longest it will be for the entire year during the winter solstice.
Ancient Solstice Sites
In the ancient world, people built a number of monuments to commemorate the solstice. One example is Newgrange, a huge Stone Age tomb mound built in the Irish countryside around 3200 B.C., about a thousand years before Stonehenge. A tunnel facing the solstice sunrise runs to a main chamber, where the dead may have once been placed. A small window bathes the chamber in solstice light for 17 minutes
The Paracas people of Peru, who lived around 800 to 100 B.C., crisscrossed the desert with lines of earth and rock called geoglyphs that connect ceremonial mounds with the place where the winter solstice sun sets on the horizon. The famed Nasca Lines—awe-inspiring monkeys, lizards, and other giant figures etched into the earth by a subsequent Peruvian culture around A.D. 1 to 700—also feature alignments with the winter solstice.
Ancient Egypt's sprawling Temple of Karnak was constructed in alignment with the winter solstice at Luxor more than 4,000 years ago. Similar alignments can be seen at Angkor Wat in Cambodia and Machu Picchu in Peru.
Christmas Connection?
For more than two billion Christians, the solstice has long been overshadowed by Christmas. But to historian David Gwynn of the University of London, the proximity of the two events may not be an accident.
According to Gwynn, one theory holds that Christmas was set on December 25 to replace a Roman holiday, which had roots in the pagan cult of Sol Invictus (“the unconquered sun”).
Other solstice traditions color today's winter holiday celebrations. Scandinavians once celebrated Juul, or Yule, a multiday feast marking the sun god's return. In Britain, Druids observed the solstice by cutting mistletoe.
Some ancient solstice celebrations continue in the present day. Iran's Yalda festival marks the day when Mithra, an angel of light, was thought to have been born. The tradition was adopted into Zoroastrianism and is still observed by staying up late and savoring treats like watermelon and pomegranate.
This year's winter's solstice arrived on Dec. 21. Here's what you need to know. (Claritza Jimenez/The Washington Post)
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 point south 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.
[Sunrise and sunset times on the winter solstice]
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!
The Capital Weather Gang takes us through their predictions for the upcoming 2017-2018 winter weather in Washington, D.C. (Joyce Koh/The Washington Post)
Read more:
This chart shows why winter should, without question, officially begin Dec. 1
GETTY - STOCK IMAGE Winter Solstice 2017 is on December 21
The annual pagan celebration, also known as midwinter, signifies the astrological start of the winter in the Northern Hemisphere. It usually occurs on or around the December 21 of each year. The winter solstice takes place when the sun reaches its most southern point over the Tropic of Cancer, which is one of main lines of latitude around the Earth. The opposite is true in the Southern Hemisphere where the day is long because the sun will be at its highest point in the sky today.
Why is today the shortest day of the year? Unlike our ancestors, we know the astronomical phenomenon not to do with our distance from the sun but the tilt of the earth and how it orbits around the sun. Earth has a tilt of about 23.5 degrees and as it moves around the Sun, the Northern and Southern hemispheres face opposite directions. This affects how much warmth and light different places get. On winter solstice, the Northern Hemisphere is leaning the most away from the sun meaning it receives the least amount of possible daylight hours and the longest night.
When is the shortest day of the year? Today is the shortest day with seven hours and 50 minutes of daylight. Sunrise was at 8.03am GMT and sunset is at 3.53pm. While the cold weather will remain going forward, you should start to notice the days will be longer after December 21. There may be other subtle changes that you see today during winter solstice, according to astronomical website EarthSky. EarthSky said: “You might notice how low the sun appears in the sky at local noon. And be sure to look at your noontime shadow. Around the time of the December solstice, it’s your longest noontime shadow of the year.”
GETTY - STOCK IMAGE Winter Solstice 2017: There will be seven hours and 50 minutes of daylight today
