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| Image via rhythmsofplay.com |
| At the December solstice, the Northern Hemisphere of Earth is leaning most away from the sun. Above the Arctic Circle, there is 24 hours of darkness. Meanwhile, the Southern Hemisphere and Antarctic are bathed in sunlight. |
EarthSky.org
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 Wednesday, December 21, at 10:44 UTC (December 21 at 2:44 a.m. PST). No matter where you live on Earth’s globe, a solstice is your signal to celebrate.
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.
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.