The earliest sunset of the year (in the northern hemisphere) is almost on us. I didn’t know, until Leonard Tramiel told me last summer around the time of the solstice, that the earliest and latest sunrise and sunset don’t occur on the solstice. Bruce McClure at EarthSky explains.
The next solstice in 2014 comes on December 21 and marks an unofficial beginning for winter in the Northern Hemisphere. For this hemisphere, this upcoming solstice brings the shortest day and longest night of the year. And yet the earliest sunsets for middle latitudes in the Northern Hemisphere happen around December 7.
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. In the month of December, the time period from one solar noon to the next is actually half a minute longer than 24 hours. 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 on December 7.
The later clock time for solar noon also means a later clock time for sunrise and sunset.
Good to know, eh?