Wednesday, December 27, 2006

Gaining Light

Sitting at latitude 61, we tend to notice the soltices more than other folk. For us it means it is no longer getting darker. So, even though it is the official first day of winter, it is a day of hope - each day it starts to get lighter. And here that difference is significant.

On December 21 we lost 7 seconds of daylight (for a total of 5 hours and 27 minutes from sunrise to sunset)

On December 22 we gained 5 seconds of daylight.
On December 23 we gained 17 seconds
December 24 - 29 seconds
December 25 - 41 seconds
December 26 - 54 seconds
Today - 1 minute and 6 seconds

By the spring equinox (March 21) we'll be gaining 5 minutes 45 seconds of sun per day! That means we gain an hour of sun in 11 days. You really notice that kind of change. Right now, the rate of gain is significant as you can see in the chart at the bottom.

Diagram above from the BBC

This is all happening because of the tilt of the earth - 23.4 degrees. Now, the northern hemisphere is tilted away from the sun, so it rises to the south, stays close to the horizon all day, then dips back down in the south.

In the summer it will tilt toward the sun. So for us, the sun will rise in the northeast, circle around to the south and then head back up to the northwest to set. On June 21, 2007 - the summer solstice the sun will rise at 4:20 AM and set at 11:43 PM. That's 19hours 22minutes from sunrise to sunset. But it isn't far beyond the horizon, so we have a loooooong twilight. At the summer solstice in June, it never really gets 'dark,' though it gets to be dusk.

Date..................Sunrise....Sunset......Hours Sun Up......Loss(-) or Gain(+)
Dec 16, 2006....10:10 AM....3:40 PM ....5h 29m 34s..− 1m 08s
Dec 17, 2006 ...10:11 AM... 3:40 PM.... 5h 28m 37s.. − 0m 56s
Dec 18, 2006... 10:12 AM... 3:40 PM.... 5h 27m 53s.. − 0m 44s
Dec 19, 2006. ..10:13 AM... 3:40 PM.... 5h 27m 21s ..− 0m 32s
Dec 20, 2006... 10:14 AM... 3:41 PM.... 5h 27m 01s.. − 0m 19s
Dec 21, 2006... 10:14 AM... 3:41 PM.... 5h 26m 54s.. − 0m 07s
Dec 22, 2006... 10:15 AM... 3:42 PM.... 5h 27m 00s.. + 0m 05s
Dec 23, 2006... 10:15 AM... 3:42 PM.... 5h 27m 17s.. + 0m 17s
Dec 24, 2006....10:15 AM... 3:43 PM.... 5h 27m 47s.. + 0m 29s
Dec 25, 2006... 10:16 AM... 3:44 PM.... 5h 28m 28s.. + 0m 41s
Dec 26, 2006... 10:16 AM... 3:45 PM.... 5h 29m 23s.. + 0m 54s
Dec 27, 2006... 10:16 AM... 3:46 PM.... 5h 30m 29s.. + 1m 06s
(Source: sun calculator)

Another way to envision all this is through a daylight map.
For more on all this - like the perihelion (day the sun and earth are closest which will be January 4 in 2007, and why the perihelion regresses in the year over time), Milankovitch cycles, and other astronomical phenomena that affect seasons and climate visit the US Naval Academy site or About's Geography site.

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