Everything you need to know: December solstice 2015
 December solstice |
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Sunlight on Earth, on the day of the winter solstice. The north polar region of Earth is in 24-hour darkness, while the south polar region is in 24-hour daylight. Gif via Wikimedia Commons.
Late dawn. Early sunset. Short day. Long night. For us in the Northern Hemisphere, the December solstice marks the longest night and shortest day of the year. Meanwhile, on the day of the December solstice, the Southern Hemisphere has its longest day and shortest night. This special day is coming up on Tuesday, December 22 at 4:48 UTC (December 21 at 10:48 p.m. CST). No matter where you live on Earth’s globe, a solstice is your signal to celebrate. Follow the links below to learn more about the 2015 December solstice.
Day and night sides of Earth on the December 2015 solstice
Day and night sides of Earth at the instant of the December 2015 solstice (2015 December 22 at 4:48 Universal Time). Note that the north polar region of Earth must endure 24 hours of night, while the south polar region gets to bask in 24 hours of daylight. Image credit: Earth and Moon Viewer
When is the solstice where I live? The solstice happens at the same instant for all of us, everywhere on Earth. In 2015, the December solstice comes on December 21 at 10:48 p.m. CST. That’s on December 22 at 4:48 Universal Time. It’s when the sun on our sky’s dome reaches its farthest southward point for the year. At this solstice, the Northern Hemisphere has its shortest day and longest night of the year.
To find the time in your location, you have to translate to your time zone. Click here to translate Universal Time to your local time.
Just remember: you’re translating from 4:48 UT on December 22. So for most of the world’s eastern hemisphere – Europe, Africa, Asia, Australia and New Zealand – the December solstice actually comes on December 22. For example, if you live in Perth, Australia, you need to add 8 hours to Universal Time to find out that the solstice happens on December 22, at 12:48 p.m. AWST (Australian Western Standard Time).
Earth has seasons because our world is tilted on its axis with respect to our orbit around the sun. Image via NASA.
What is a solstice? The earliest people on Earth knew that the sun’s path across the sky, the length of daylight, and the location of the sunrise and sunset all shifted in a regular way throughout the year. They built monuments such as Stonehenge in England – or, for example, at Machu Picchu in Peru – to follow the sun’s yearly progress.
But we today see the solstice differently. We can picture it from the vantage point of space. Today, we know that the solstice is an astronomical event, caused by Earth’s tilt on its axis, and its motion in orbit around the sun.
Because Earth doesn’t orbit upright, but is instead tilted on its axis by 23-and-a-half degrees, Earth’s Northern and Southern Hemispheres trade places in receiving the sun’s light and warmth most directly. The tilt of the Earth – not our distance from the sun – is what causes winter and summer. At the December solstice, the Northern Hemisphere is leaning most away from the sun for the year.
At the December solstice, Earth is positioned in its orbit so that the sun stays below the north pole horizon. As seen from 23-and-a-half degrees south of the equator, at the imaginary line encircling the globe known as the Tropic of Capricorn, the sun shines directly overhead at noon. This is as far south as the sun ever gets. All locations south of the equator have day lengths greater than 12 hours at the December solstice. Meanwhile, all locations north of the equator have day lengths less than 12 hours.
For us on the northern part of Earth, the shortest day comes at the solstice. After the winter solstice, the days get longer, and the nights shorter. It’s a seasonal shift that nearly everyone notices.
Around the time of the winter solstice, watch for late dawns, early sunsets, and the low arc of the sun across the sky each day. Notice your noontime shadow, the longest of the year. Photo via Serge Arsenie on Flickr.
Meanwhile, at the summer solstice, noontime shadows are short. Photo via the Slam Summer Beach Volleyball festival in Australia.
For all of Earth’s creatures, nothing is so fundamental as the length of daylight. After all, the sun is the ultimate source of all light and warmth on Earth.
If you live in the northern hemisphere, you can notice the late dawns and early sunsets, and the low arc of the sun across the sky each day. 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.
In the Southern Hemisphere, it’s opposite. Dawn comes early, and dusk comes late. The sun is high. It’s your shortest noontime shadow of the year.
EarthSky Facebook friend John Michael Mizzi saw this sunset from the island of Gozo (Malta), south of Italy. The earliest sunsets come a couple of weeks before the winter solstice.
Why doesn’t the earliest sunset come on the shortest day? The December solstice marks the shortest day of the year in the Northern Hemisphere and longest day in the Southern Hemisphere. But the earliest sunset – or earliest sunrise if you’re south of the equator – happens before the December solstice. Many people notice this, and ask about it.
The key to understanding the earliest sunset is not to focus on the time of sunset or sunrise. The key is to focus on what is called true solar noon – the time of day that the sun reaches its highest point, in its journey across your sky.
In early December, true solar noon comes nearly 10 minutes earlier by the clock than it does at the solstice around December 22. With true noon coming later on the solstice, so will the sunrise and sunset times.
It’s this discrepancy between clock time and sun time that causes the Northern Hemisphere’s earliest sunset and the Southern Hemisphere’s earliest sunrise to precede the December solstice.
The discrepancy occurs primarily because of the tilt of the Earth’s axis. A secondary but another contributing factor to this discrepancy between clock noon and sun noon comes from the Earth’s elliptical – oblong – orbit around the sun. The Earth’s orbit is not a perfect circle, and when we’re closest to the sun, our world moves fastest in orbit. Our closest point to the sun – or perihelion – comes in early January. So we are moving fastest in orbit around now, slightly faster than our average speed of about 30 kilometers (18.5 miles) per second. The discrepancy between sun time and clock time is greater around the December solstice than the June solstice because we’re nearer the sun at this time of year.
Solstice Pyrotechnics II by groovehouse on Flickr.
The precise date of the earliest sunset depends on your latitude. At mid-northern latitudes, it comes in early December each year. At northern temperate latitudes farther north – such as in Canada and Alaska – the year’s earliest sunset comes around mid-December. Close to the Arctic Circle, the earliest sunset and the December solstice occur on or near the same day.
By the way, the latest sunrise doesn’t come on the solstice either. From mid-northern latitudes, the latest sunrise comes in early January.
The exact dates vary, but the sequence is always the same: earliest sunset in early December, shortest day on the solstice around December 22, latest sunrise in early January.
And so the cycle continues.
Bottom line: In 2015, the December solstice comes on December 21 at 10:03 p.m. CST. That’s December 22 at 4:48 UT. It marks the Northern Hemisphere’s shortest day (first day of winter) and Southern Hemisphere’s longest day (first day of summer). Happy solstice, everyone!
