Sunday, December 5, 2010
wondering as I often do
I always find something mysterious about the very early sunsets at this time of year. And equally I finding it mysterious that as the winter deepens it frigid grasp the days start getting longer,
Wondering about his as often do I feel sure that our ancient ancestors did as well, and perhaps they even began their celestial observations because of it, perhaps.
So here is some information from http://earthsky.org/with photos by Peter Bowers.
The 2010 solstice comes on December 21, but the earliest sunsets for the northern hemisphere are around now.
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 the actual days – 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 at 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
The exact date for the earliest sunset or earliest sunrise varies by latitude. At present, mid-temperate latitudes in the northern hemisphere have their earliest sunsets, while the southern hemisphere’s mid-temperate latitudes are waking up to their earliest sunrises. At latitudes closer to the equator, the earliest sunset or earliest sunrise has already come and gone. Closer to the Arctic and Antarctic Circles, the earliest sunset and earliest sunrise have yet to come.
As you might have guessed, the latest sunrises and sunsets aren’t on the day of the solstice either.
So there’s variation in the exact dates, but the sequence is always the same for both hemispheres. First: earliest sunset before the winter solstice, the winter solstice itself, latest sunrise after the winter solstice. Half a year later: earliest sunrise before the summer solstice, the summer solstice itself, latest sunset.
The earliest and latest sunsets and sunrises are lovely phenomena that happen around every solstice. People around the world notice them and often ask about them
The winter solstice is the shortest day, but the earliest sunsets come a couple of weeks before the solstice, in early to mid-December, depending on your latitude. What’s more, the latest sunrises come after the winter solstice, in January.
Why isn’t the earliest sunset on the year’s shortest day? It’s because a clock ticks off exactly 24 hours from one noon to the next, while the actual days – as measured by the spin of the Earth – are rarely exactly 24 hours long.
One Earth spin can be measured from what is called one “solar noon” or “midday” to the next. Solar noon 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. So – two weeks before the solstice, for example – the sun reaches its noontime position at 11:52 a.m. local standard time. Two weeks later – on the winter solstice – the sun reaches its noontime position at 11:59 a.m. That’s 7 minutes later.
The later clock time for solar noon also means a later clock time for sunrise and sunset. The result: earlier sunsets before the solstice and increasingly later sunrises for a few weeks after the solstice.
By the way, the exact date of earliest sunset varies with latitude. But the sequence is always the same: earliest sunset in early December, winter solstice, latest sunrise in early January
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