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Why our sunset time is getting later, even before the winter solstice?

A viewer asked, so Chester Lampkin went searching for an answer!
Credit: Getty Images

ST. LOUIS -- A viewer wanted to know why our sunset time is already getting later, even though we haven’t hit the winter solstice. Does this mean the Winter Solstice is not the shortest day of the year?

I set out to find an answer.

According to the U.S. Naval Observatory, the winter solstice is the shortest day of the year for us. The sun appears at its lowest in our sky on the winter solstice, leading to the shortest daylight of the year. But due to a number of factors, the earliest sunset time of the year comes around December 6th in St. Louis.

The main reason this happens is that our clocks run on a 24-hour day, while the true Earth day is only 23 hours and 56 minutes. This discrepancy, along with the tilt of the Earth’s axis, causes our earliest sunset to occur before the Winter Solstice. The latest sunrise of the year is offset too, occurring on or about January 5th.

By the way, this also why we have a leap day every four years! That’s the day we make up for lost time.

St. Louis' latest sunrise and earliest sunset is unique to our latitude. The closer you are the poles, the closer the extreme sunrise and sunset times are to the solstice. 

Curious about the difference between mechanical clock time and the true solar time? You can read more about that here. 

So we can verify that the winter solstice is, in fact, the shortest daylight of the year in St. Louis.

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