Why Isn't the Earliest Sunset on the Winter Solstice?

Sunrise and sunset for Springfield in December

Winter officially starts on December 21st this year, and is considered the day with the shortest amount of sunlight. However, the earliest sunset actually occurs before the solstice. How come these don't line up?

While the length of a day is defined as 24 hours long, in reality, it varies slightly due to the spin of the Earth. The actual length is defined as a "solar day," and its length can vary from a few seconds to 30 seconds longer than 24 hours. Additionally, latitude plays a role. At our latitude (around 40 degrees North), the earliest sunset occurs during the first several days of December, then begins to get later starting around the 14th. However, during this time, the sunrise steadily gets later, and doesn't reach its latest point until around New Year's Day.

The times on this chart are specifically for Springfield. Interested in calculating the sunrise and sunset for your area? Visit the NOAA Solar Calculator website at https://www.esrl.noaa.gov/gmd/grad/solcalc/ .