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: The 2012 December solstice takes place on Friday, December 21 at 11:12 Universal Time. That’s 5:12 a.m. Central Standard Time. Happy solstice!

(photo by John Michael Mizzi, who saw this sunset from the island of Gozo (Malta), south of Italy)