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?
The only thing I can imagine is that this is due to the fact that the Earth is both rotating and moving in orbit around the sun. Am I correct?
I believe the term you’re looking for is analemma. It’s caused by the fact that the Earth’s orbit is elliptical, not circular, thus does not travel at the same speed along its orbit, which makes the length of the solar day vary over the course of the year.
OK, I mangled the grammar on that last comment, but I think it’s clear enough.
moarscienceplz, what you may be thinking of is the difference between the tropical year (the time it takes for the Earth to reach the same position wrt the sun, ie solstice to solstice), vs. the sidereal year (the time it takes for the Earth to orbit the sun once wrt the celestial sphere).
Even more relevant, perhaps: The equation of time is the part of the analemma that is responsible for the sun not being in the south at the same time every day. It is indeed due to the earth’s elliptical orbit. As Kepler found out (and Newton explained), the line between the sun and the earth sweeps out equal areas over equal time intervals, which implies a slower angular rate when the distance is larger (near aphelion).
So when is the latest sunrise?
From Wikipedia entry Analemma:
Ahhh, now I understand it better. Thanks, Eamon Knight. Also, thank you, Anthony K and Harald Hanche-Olsen. I learned several things today.
Whatever you are celebrating this month, always remember: Axial tilt is the Reason for the Season.
All I know is that it gets dark at 4:30, and now that I’m old and my eyes don’t work as well, I hate driving in it. But yeah, I was aware of the discrepancies.