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Thursday's Winter Solstice Is More Than the Longest Night of the Year It's all in our tilt.
If you’ve ever asked your friend to explain what the winter solstice is, you’ve probably gotten a response along the lines of: “It’s the longest night of the year!” While this is true — the sun is in the sky for the shortest amount of time of the year — the winter solstice is also a key moment in the changing of our seasons.
If you’re living in the Northern Hemisphere and have had a lot of time on your hands, you’ve probably noticed that the sun looks like it’s getting smaller and moving south. Well, as you know, the Earth orbits around the sun, so it’s actually us doing the moving, but still.
The Earth revolves around the sun at a slight angle — 23.44 degrees to be exact. This tilt gives us all four of our seasons. When the Northern Hemisphere is tilted towards the sun, its rays are hitting the top half of our planet directly, which means it’s summer up here. When the opposite is the case, it’s winter, like right now.
The winter solstice, which is happening on December 21, is the time when the Northern Hemisphere has tilted as far away from the sun as possible. This means the sun will set earlier than it has all year long, making for a seriously extended night. It’s perfect if you want to catch up on some snoozing, or just really like the darkness.
From the 21st forward, the Northern Hemisphere will slowly tilt back towards the sun, ushering in spring and, eventually, summer. The sun will once again begin rising higher and appearing bigger in the sky.
For us, this yearly occurrence is nothing more than a really long night and maybe some complaints about the sun setting way too soon. But for ancient civilizations, this time of the year was cause for huge celebration because it marked the “rebirth” of the sun.
Civilizations across Europe would get together and feast, Druids would gather to dance and sing, and the Mayans built pyramids commemorating this time of the year.
So why not commemorate this transitional time of year by throwing a solstice rager? You’ll have plenty of dark hours to throw down.
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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.
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.
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!
Read more:
This chart shows why winter should, without question, officially begin Dec. 1
Compiled by Julia Hatmaker | jhatmaker@pennlive.com
The winter solstice is Dec. 21 for the northern hemisphere -- but what exactly is the winter solstice? And why is it important?
Read on to find out the definition of the solstice, traditions for celebrating it and how long, exactly, is the shortest day of the year.
