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On Dec. 21, or Thursday this year, the sun will hug the horizon. For those of us in the Northern Hemisphere, it will seem to barely rise — hardly peeking above a city’s skyline or a forest’s snow-covered evergreens — before it swiftly sets.
For months, the orb’s arc across the sky has been slumping, shortening each day.
In New York City, for example, the sun will be in the sky for just over nine hours — roughly six hours less than in June at the summer solstice. The winter solstice marks the shortest day of the year, before the sun reverses course and climbs higher into the sky. (At the same time, places like Australia in the Southern Hemisphere mark the summer solstice, the longest day of the year.)
This is a good opportunity to imagine what such a day might look like if we had evolved on another planet where the sun would take a different dance across the sky. You might want to feel thankful for the solstices and seasons we do have, or we might not be here to witness them at all.
The solstices occur because most planets do not spin upright, or perpendicular to their orbits.
The Earth, for example, slouches 23.5 degrees on a tilted axis. This leaves the planet’s North Pole pointed toward the North Star over relatively long periods of time, even as Earth makes its yearlong migration around the sun. That means the Northern Hemisphere will spend half the year tilted slightly toward the sun, bathing in direct sunlight during summer’s long, blissful days, and half the year cooling off as it leans slightly away from the sun during winter’s short, frigid days. Dec. 21 marks the day when the North Pole is most tilted away from the sun.
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But every planet slouches at different angles.
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The axial tilt of Venus, for example, is so extreme — 177 degrees — that the planet is essentially flipped upside down with its South Pole pointing up. Perhaps counter-intuitively, that means that there’s very little tilt to its upside-down spin and its hemispheres will never dramatically point toward or away from the sun. As such, the sun’s dance across the sky will remain relatively stable — shifting by a mere six degrees over the course of a Venusian year.
00:35 Why Winter Should Start December 1st Meteorologist Domenica Davis explains why meteorological winter starts on December 1st
At a Glance The winter solstice is just a moment of time, astronomically speaking.
The least amount of possible daylight hours occurs on the winter solstice.
On Thursday, Dec. 21, 2017, the winter solstice marks the official astronomical start of winter in the Northern Hemisphere.
However, many of you already have seen at least some cold and snow for weeks prior to the winter solstice.
(MORE: Winter Storm Central )
So let's walk through what the winter solstice actually means.
Orientation of the sun's most direct rays relative to the Earth's tilt around the time of the Northern Hemisphere's winter solstice.
A Tilted Earth Gives Us Our Seasons
Looking from space, the Earth isn't aligned in such a way that a line going through the planet's poles is vertical.
Rather, the Earth has a tilt of roughly 23.5 degrees off a vertical axis.
Because of that tilt, the Northern and Southern hemispheres are tilted either toward or away from the sun at different times of the year as the Earth makes its orbit around its closest star.
As a result, the most direct sunlight shifts between a band of latitudes, specifically between the Tropic of Cancer and Tropical of Capricorn, throughout the year, leading to the change of seasons seen in the middle and higher latitudes.
Typical possible daylight hours at the December (Northern Hemisphere's winter) solstice.
The Solstice: One Moment in Time
Typically around Dec. 21 or 22 – though on rare occasions, it can be as early as Dec. 20 or as late as Dec. 23 – the sun is directly overhead at the Tropic of Capricorn (23.5 degrees south latitude).
That precise instant in time – in 2017, at 11:28 a.m. EST on Dec. 21 – is the solstice.
(MORE: Why Winter Should Really Start on December 1 )
Around the time of the winter solstice, the sun is farthest south in the sky at noon in the Northern Hemisphere. Less direct solar radiation means less heating of the ground in the winter hemisphere.
The day of the winter solstice features the least amount of possible daylight hours and the longest night in the Northern Hemisphere. The reverse is true in the Southern Hemisphere.
Below are some sunrise and sunset times for the December 2017 (Northern Hemisphere winter) solstice:
Sunrise Sunset Possible Daylight Time New York 7:16 a.m. 4:32 p.m. 9 h 16 m Miami 7:03 a.m. 5:35 p.m. 10 h 32 m Chicago 7:15 a.m. 4:23 p.m. 9 h 8 m Dallas 7:26 a.m. 5:25 p.m. 9 h 59 m Los Angeles 6:55 a.m. 4:48 p.m. 9 h 53 m Fairbanks 10:58 a.m. 2:41 p.m. 3 h 43 m
Soon after the winter solstice, the number of daylight hours in the Northern Hemisphere will gradually grow longer each day until the summer solstice in late June, when the greatest amount of daylight hours occurs.
(MORE: Winter, Early-Spring Temperature Outlook )
Today is winter solstice, the shortest day of the calendar year in the UK with just seven hours and 49 minutes of daylight.
This astronomical phenomenon occurs every year between 19 and 21 December in the run-up to Christmas as the northern hemisphere is tilted on the earth's axis at its furthest remove from the sun during our planet's 365-day progression around the solar system.
With the celestial body’s rays cast on the distant Tropic of Capricorn while the North Pole lies cloaked in shadow, the inhabitants of the northern hemisphere find themselves doomed to several months of long dark nights and low temperatures.
The southern hemisphere’s equivalent event takes place in mid-June, at which point we are enjoying our summer solstice, the longest day at the height of British summertime.
All this means the UK experienced a late sunrise at 8.04am this morning and will see an early sunset at 3.53pm this afternoon.
Historically, the solstice has been used to mark midwinter, the official start of winter, after which point the nights gradually begin to grow shorter as we approach the spring equinox on 20 March.
Druids still gather at Stonehenge in the West Country to mark the occasion to this day, pagan revellers meeting at dawn to observe the morning sun rising over the ancient circle’s Heel Stone.
Although the day symbolises the death of the old year, this is by no means a solemn occasion, with druids celebrating the renewal of life and rejoicing in a moment of seasonal rebirth.
Winter Solstice celebrations around the World
6 show all Winter Solstice celebrations around the World
1/6 Beijing, China Models painted in camouflage colours to blend in with the background pose for Chinese artist Liu Bolin's artwork "Dongji" for the Winter Solstice
2/6 Seoul, South Korea Employees at the Korean Folk Village, hand out bowls of red bean soup to visitors at the village in Yongin, south of Seoul, South Korea, to celebrate the upcoming winter solstice.
3/6 Riga, Latvia People wear costumes as they participate in the ancient Yule Log dragging tradition during winter solstice celebrations in Riga, Latvia,
4/6 Toronto, Canada A performer blows fire from her mouth at the 26th Annual Kensington Market Winter Solstice Parade in Toronto
5/6 Berlin, Germany A cyclist is silhouetted against the sky as he waits for the light to change at Berlin's Potsdamer platz .The winter solstice has seen unusually high temperatures this year.
6/6 Wiltshire, England People gather at Stonehenge in Wiltshire on the Winter Solstice to witness the sunrise on the shortest day of the year.
In Germany and Scandinavia, a 12-day solstice (“Yule”) is observed from mid-December, to which the rest of Europe and North America owes many of its Christmas customs, from the tree and front door wreath to the chocolate log. Like the Celtic druids in the UK, these traditions emphasise the natural world and its vital capacity for self-renewal.
Interestingly, the Western winter solstice is also observed in Iran, where families gather on “Yalda night” to eat nuts, pomegranates and watermelons and read poetry together in honour of the longest night.
There are sunnier days ahead! Say goodbye to shorter days, and ever so slowly start to welcome back more sun as the winter solstice arrives on Thursday.
The winter solstice is the shortest day and longest night of the year in the Northern Hemisphere, while the Southern Hemisphere experiences just the opposite.
CBS News asked University of Massachusetts astronomer Stephen Schneider to explain the astronomical cause of the solstice, and he answered seven basic questions about the phenomenon that everyone should know.
What is the winter solstice, and why do we have it?
The winter solstice occurs at the moment the Earth's tilt away from the sun is at a maximum.
"Earth's axis is tilted 23.5 degrees relative to its orbit, and on December 21, Earth will be at the point in its orbit when the North Pole is tilted at its maximum away from the sun," Schneider told CBS News. "The effect of this in the Arctic (within 23.5 degrees of the North Pole) is completely in darkness as Earth spins that day."
For those of us in the Northern Hemisphere, it's the day with the shortest amount of daylight — less than 12 hours — and our longest night of the year. However, that's not the case for everyone. While it's winter for those of us in the Northern Hemisphere, people in the Southern Hemisphere experience it as their summer solstice with the longest stretch of daylight.
When does the winter solstice occur?
The Earth's North Pole will be tilted farthest from the sun at 11:27 a.m. ET on Thursday, Dec. 21, 2017, according to the National Weather Service. For a complete listing of the dates of the winter and summer solstices and the spring and fall equinoxes through 2025, check out this calendar from the U.S. Naval Observatory.
Why isn't the winter solstice on the same day each year?
The date of the winter solstice varies from year to year for people in different time zones. It typically occurs around Dec. 21 or 22, though on rare occasions it can be as early as Dec. 20 or as late as Dec. 23, according to the Weather Channel.
That's because our calendars aren't a precise match to the solar year.
Schneider writes: "Earth takes about 365 1/4 days to orbit the Sun. Next year, the moment of the solstice will be about 6 hours later at 5:23 p.m. ET. In 2019, it will be at 11:19 p.m. ET, so for people living in Puerto Rico and other time zones to the east, the date of the solstice will be on the 22nd."
The 2017 Winter Solstice arrives tomorrow - almost in lockstep with the incoming cold air mass expected to drop temperatures for the holidays. #WednesdayWisdom pic.twitter.com/Jb2DEfwDWj — NWS (@NWS) December 20, 2017
Is the winter solstice the coldest day of the year?
The winter solstice is not normally the coldest day of the year. There is actually a lag between the shortest day of the year and the coldest average temperatures, the National Weather Service reports.
"The coldest time usually comes a month or two later because even though the amount of solar heating is beginning to increase, it isn't yet enough to reverse the cooling," Schneider says. "This effect, called 'the lag of the seasons,' is similar to how when you turn down the heat while you're cooking — the pot won't immediately reach its coolest temperature."
Will shadows on the day of the solstice be longer?
Noontime shadows on Thursday will be longest for those of us in the Northern Hemisphere because the sun doesn't rise as high in the sky. The closer you are to the Arctic, the longer the shadows (and the shorter your day) will be.
"If you live 23.5 degree south of the equator (the Tropic of Capricorn), the sun will pass straight overhead at noon on December 21, and people south of that will have their shortest noontime shadows of the year," Schneider says.
Why doesn't the earliest sunset come on the shortest day?
The winter solstice may be the shortest day of the year, but that doesn't mean you'll have an early sunset. The exact date of the earliest sunset depends on your latitude, so it's not the same for everyone.
If you live in the southernmost U.S., your earliest sunsets are in late November, according to EarthSky.org. If you're farther north, your earliest sunsets are around December 7.
"We're actually closer to the sun this time of year (also a surprise to many people) and moving faster in our orbit, so the Earth has to turn a little extra to face the sun each day compared to other times of year," Schneider says. "The extra 30 seconds of a solar day makes true solar noon — and sunrise and sunset — about 30 seconds later each day."
What does all of this have to do with Stonehenge?
For more than 4,000 years, Stonehenge has stood like a Neolithic sundial, marking the longest day of the year and the shortest — taking aim at the sun like a giant stone gunsight.
Every year, people flock to the ancient monument in Wiltshire, England, to watch the sunrise during the winter solstice, when the structure is directly aligned toward the sun.
"For thousands of years people built structures to track the changing position of the sun, and of course the solstices were some of the most notable dates because the sun would reach its extreme north or south positions before reversing course for the next six months," Schneider says. "Ancient structures that allow you to track the shifting position of the sun between its extremes are found all over the world. Stonehenge is one of the larger of these structures begun perhaps 5,000 years ago with its main axis aligned toward sunrise on the June solstice."
