As a new Pennsylvania resident, I was shocked by how early the sun set in December. Coming from Florida, the earliest sunset I had experienced was around 5:45 pm. At AGI headquarters however, the sun sets earlier and earlier until it was setting close to 4:30 pm.
I was curious to see exactly how early this sunset was and looked up a graph of sunrise and sunset times in Philadelphia, PA for the entire year. My observations were confirmed as the earliest sunset on the graph was 4:35 pm, but I was surprised to see that the date of this sunset was December 6th. I had always thought the shortest day of the year would also have the earliest sunset. Despite having no basis for my claim, I decided to validate this graph using STK.
In STK, I inserted Philadelphia as a place object and pulled the sunlight intervals into MATLAB. From there, I graphed the data across the entire year (accounting for daylight savings time) and produced a plot that is nearly identical to the one I had found online.
With both graphs confirming that the earliest sunset is on December 6th, over two weeks before the winter solstice, I was curious why. According to Bruce McClure on EarthSky.org, this difference is due to how we measure the length of a day. Our clocks assume a day is exactly 24 hours, but the actual length of a day (the time it takes Earth to complete one revolution) is rarely the same.
This discrepancy is shown through solar noon. This is the time that the sun is at its highest point during the day. For example, in Philadelphia on December 6th, solar noon is at 11:52 am while solar noon on Dec 21st is 11:58 am. A later solar noon means the sun will rise and set at a later clock time, explaining why the earliest sunset does not fall on the shortest day of the year.
The latitude of a location also matters for determining the date of the earliest sunset. Places in the Northern Hemisphere closer to the equator will experience the earliest sunset of the year in November, while places closer to the poles experience it closer to the solstice due to the declination of the sun.
So now when the days are getting shorter in the winter or longer in the summer, you will know why. And thanks to STK, no one should make the same mistake I did in assuming the earliest sunset is on the shortest day of the year.