Earliest Sunset was Tonight: Happy Seculus Eve!

Reader Howard Wilk wrote to remind me that while the winter solstice is still almost two weeks away, tonight we in Philadelphia and others at our latitude will see the earliest sunset of the year. He has decided to name tomorrow a new holiday in honor of the now-lengthening evenings:   

"The winter solstice marks (for us) the shortest day (light hours), but it's not our date this Dec.-Jan. for the latest sunrise, which will be 5 January, or our date for the earliest sunset, which is today, Thursday, 8 December. Since night owls like me routinely experience sunset but not sunrise, today is effectively the shortest day for us. With a clock but without specific astronomical knowledge one doesn't know one has reached the day of the earliest sunset until the next day, when the sun sets later than the day before. That's a cause for celebration and I call that day, the day after the day of the earliest sunset, Seculus. Seculus is a secular holiday completely divorced from religion unlike Christmas, Hanukkah, Saturnalia, etc., or even Festivus (which is defined as 23 December, two days before Christmas)."

This report in Earthsky.org backs Wilk on the date. The sun set tonight at 4:35 and on the Winter Solstice, which is Dec. 22 this year, it will set at 4:39. Earthsky has a nice explanation for this gap between Seculus and Solstice:

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, the sun reaches its noontime position at 11:52 a.m. local standard time. Two weeks from now – on the winter solstice – the sun will reach its noontime position around 11:59 a.m. That’s 7 minutes later than today.

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