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December bright spot: Sunsets start moving later on Dec. 15

Earth's elliptical orbit and axial tilt produce later sunsets before the winter solstice

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December sun at the Weather Lab
December sun at the Weather Lab
Paul Huttner/MPR News

Many Minnesotans could use a little positive weather news in December. Here’s one small data point that brings a little brightness into the Weather Lab this time of year. 

The shortest total daylight arrives with the winter solstice on December 21. But our Minnesota sunsets actually start getting later again before the winter solstice.  

Our earliest sunset times of the year in Minnesota peak starting this week. The sun will set in the Twin Cities at 4:31 p.m. starting this Thursday, Dec. 5. Sunset time stays at 4:31 p.m through Dec. 14. 

Then, starting on Dec. 15 about a week before the winter solstice, sunset times in Minnesota start shifting ever so slightly later at 4:32 p.m.

Check out the sun chart below from timeadndate.com.

December sun data for Minneapolis
December sun data for Minneapolis
timeanddate.com

Later before the winter solstice?  

The reason we observe later sunset times before the winter solstice is due to the earth’s elliptical orbit and axial tilt.

Earth's orbit and seasons
Earth's orbit and seasons
NASA

It’s complicated, but quirks in the timing of sunrise and sunset around the solstices mean the latest sunrise times occur after the winter solstice. The earliest sunset times occur before the winter solstice. 

Here’s more from timeadndate.com

If you look at the sunrise and sunset times for any city in the Northern Hemisphere around the December solstice, you will notice that the earliest sunset occurs a few days before the solstice and the latest sunrise happens a few days after the solstice.

Winter sun at the Weather Lab
Winter sun at the Weather Lab
Paul Huttner/MPR News

So why do our earliest sunsets occur before the winter solstice?? Here’s a good description from Deborah Byrd at EarthSky radio.

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 Dec. 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 18.5 miles per second (30 kilometers 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.

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.

December brings the shortest daylight of the year in Minnesota. One bright spot we can observe is that sunset times actually start moving later starting on Dec. 15. By Dec. 31, the sunset time in the Twin Cities occurs at 4:41 p.m. That’s a full 10 minutes later than our peak early sunset time of 4:31 p.m. starting later this week. 

So, there are some bright silver linings in our darkest month of Minnesota winter.