Why our sunsets are stuck in time
Sunset hits Seattle at 4:18 p.m. through Dec. 17 — the earliest time of the year — though the shortest day of the year isn't until several days later.
Seattle Times staff reporter
Now is our hibernation time, when the sun tucks us in for the night at 4:18 p.m., the earliest sunsets of the year.
By 3:30 in the afternoon through Dec. 17, the sun already is taking on that apricot glow, coloring the sky and west-facing buildings downtown gold, their windows flaring orange with reflected sunset light.
A dramatic darkling is under way long before dinner, before it's time to go home for the day, as night presses in and the Olympics occlude to the west. Holiday lights and shop windows burn in the gathering dark, all the warmer and prettier for the contrast, especially when the streets shine wet with rain.
Sunrise also comes later every morning. By the time winter officially begins Dec. 22, we will be down to the shortest day of the year. A mere stub of a day, out almost before it is lit, with a mere eight hours of daylight.
This seems odd: Sunrise is coming later, even as sunset is stuck at the same time of day for more than a week. And, despite the continued decrease in daylight hours until Dec. 22 (due to later and later sunrises), the sun actually starts setting later as of Dec. 18.
What's going on?
Dale Durran, chairman of the atmospheric sciences department at the University of Washington, says it's a combination of astronomical effects going on at the same time. At the heart of the matter are the positions of the sun and Earth relative to each other.
One of the effects of their changing alignment makes for a difference between clock time and solar time. Noon, it turns out, isn't when you think it is.
It's only high noon — when the sun is directly overhead in its highest position of the sky at noon on the clock — four times a year. The high-noon position of the sun is either behind or ahead of clock time the rest of the year.
That's because of the variable rate of time it takes during the course of the year for Earth to turn all the way around to face the sun in the same place — say solar noon.
The amount of time it takes for the Earth to face the sun in the same spot varies over the course of the year, resulting in an accumulated difference between clock time and solar time by as much as 16 minutes. That would tend to make sunsets later.
But another effect is in play: Earth rotates on a tilted axis. That means the North Pole leans progressively farther from the sun until the arrival of the winter solstice on Dec. 22. That would tend to make the sun rise later and set earlier until then. Yet, because of the combination of the two effects, the time of sunset in Seattle holds steady between Dec. 5 and Dec. 17.
The gradual change in daylight length and difference between clock and solar time cancel each other out. Voilà: Sunset is parked, even as the sun continues to rise ever later.
If all this — the equation of time, the movement of the cosmos, and more — is a bit daunting, you always can take the wake-me-when-it's over approach.
It seems to work for bears.
Lynda V. Mapes: 206-464-2736
On Twitter @lyndavmapes.
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