It’s ten past four, nearly sunset. Leonard Tremiel informed me the other day that the earliest sunset is actually two weeks before the solstice, and the latest sunrise two weeks after it. He recommended Earth and Sky’s explanation.
It seems paradoxical. At middle latitudes in the U.S. – and throughout the Northern Hemisphere – the earliest sunsets of the year come about two weeks before the solstice and the shortest day of the year.
Why isn’t the earliest sunset on the year’s shortest day? It’s because of the discrepancy between the clock and the sun. A clock ticks off exactly 24 hours from one noon to the next. But an actual day – as measured by the spin of the Earth, from what is called one “solar noon” to the next – rarely equals 24 hours exactly.
Solar noon is also called simply “midday.” It refers to that instant when the sun reaches its highest point for the day. At this time of year, the time period from one solar noon to the next is actually half a minute longer than 24 hours. Today, on December 7, the sun reaches its noontime position at 11:52 a.m. local standard time. Two weeks later – on the winter solstice – the sun will reach its noontime position around 11:59 a.m. That’s 7 minutes later than today.
The later clock time for solar noon also means a later clock time for sunrise and sunset. The table below helps to explain.
For Philadelphia, Pennsylvania
Date Sunrise Solar Noon (Midday) Sunset Daylight Hours December 7 7:09 a.m. 11:52 a.m. 4:35 p.m. 9 hours 26 minutes December 21 7:19 a.m. 11:59 a.m. 4:39 p.m. 9 hours 20 minutes
Cool huh? Read the rest there, complete with sunset photos.