How much daylight will your patch of the planet receive on Thursday, Dec. 22, the date of the 2011 winter solstice?

And while no day is shorter than Thursday, why is it that sunset times have hardly budged since early December?

First, here's how much time the sun will spend above the horizon on Thursday in 25 randomly selected cities where REI operates stores: 

Source: Timeanddate.com, sunrise and sunset calculator. Note: The 2011 winter solstice occurs at approximately 12:30 a.m. EST on Dec. 22 (9:30 p.m. PST, Dec. 21).

But what about those strangely stagnant sunset times? Here in Seattle, REI's home turf, the year's earliest sunset, 4:18 p.m., first occurred on Dec. 5. (Yep, we know, we know; 4:18 p.m. is mighty early for a sunset.) Through Dec. 16, that time never changed. Since then sunset times have actually occurred later. On Dec. 22, the sun will set at 4:21 p.m.

Even so, all that time the sun has been rising later and later. It came up at 7:41 a.m. on Dec. 5, but on Thursday it won't rise until 7:55 a.m.

Move closer to the equator and the same pattern is found. On Dec. 1 in Houston, for example, sunset occurred occurred at 5:22 p.m. On Thursday it occurs 5 minutes later. Yet the sun will rise 14 minutes later than it did on Dec. 1.

Why is this? Two factors:

• Earth's elliptical orbit. A difference exists between "solar noon" (aka "local noon"), the time each day the run reaches its highest point in the sky, and what we nonastronmers call straight-up high noon (when our clocks show 12:00). In Seattle, solar noon happens on Dec. 22 at 12:08 p.m.; in Houston, 12:20 p.m. Because the earth travels in an elliptical orbit, not a circular pattern, solar noon occurs later in the day between October and April.

• The tilt (23.5 degrees) of the earth's axis. The North Pole will be as far from the sun as it gets on Thursday. While solar noon grows later, this gradual, progressive tilting of the earth's axis through the solstice time frame keeps the sunset times in place. Meanwhile, sunrise times occur later and later. The latest sunrise time in Seattle, for example, is 7:58 a.m. (Dec. 30-Jan. 4). Yet by Jan. 4, though, sunset will occur at 4:31 p.m., and the amount of daylight will have expanded by more than 8 minutes.

Explanations for this phenomenon are offered in an academic overview from Cornell University's Ask an Astronomer service, from earthsky.org, and in an article published in The Seattle Times.

However the planet does it, I'm just happy to know daylight hours will soon be back on the increase. The payoff here in the Northwest comes in early summer, when evening afterglow lingers long in the high country. So even if I stretch a hike later than wisdom dictates (standard procedure for me), I can usually keep your headlamp tucked away until 10 p.m. Good times. C'mon, summertime.

Photo above: Ocotillo silhouetted by a sunset at Big Bend National Park in Texas, by Jack Hamann. Below: Sunset at Zion National Park by REI's T.D. Wood.