The December solstice marks the longest night in Northern Hemisphere and longest day in the Southern Hemisphere. This year the December solstice comes on Dec. 21 at 5:30 pm, Central Standard Time. (On Universal Time, it is December 21 at 23:03.)
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.
Where should I look to see signs of the solstice in nature? Everywhere.
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.
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 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 21. 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 earliest sunset and the 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 18 miles per second.
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 21, latest sunrise in early January.
And so the cycle continues.
Bottom line: In 2014, the December solstice comes on December 21 at 5:03 p.m. CST. That’s December 21 at 23:03 UT. Happy solstice, everyone!”