The winter solstice is this week for us in the northern hemisphere. After the winter solstice, the days will get longer. Celebration time!
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.
If you live in this hemisphere, it’s your signal to celebrate. The shortest day is here! After the winter solstice, the days will get longer, and the nights shorter. It’s a seasonal shift that nearly everyone notices.
The solstice happens at the same instant for all of us, everywhere on Earth. But our clocks say different times.
In 2011, the December solstice takes place on Wednesday, December 21 at 11:30 p.m. CST (Thursday, December 22 at 5:30 UTC).
In 2011, all time zones to the east of the Central Standard Time meridian (90 degrees west longitude), the December solstice falls on Thursday, December 22.
The earliest humans 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, 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. That’s what causes winter and summer.
At the December solstice, Earth is positioned in its orbit so that the sun stays 23-and-a-half degrees 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 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.
Many people have an unfavorable response to this time of short days and long nights. Some people try special lighting. Others take vacations to more southerly latitudes.
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 – seems to defy logic when it happens before the solstice.
How does this happen?
The key to understanding it 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.
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