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December's Dark Days
12/20/2002
Listen in RealAudio 
We know the day of least light each year falls on the Winter Solstice, December 21. But you
might be surprised to learn that neither the latest sunrise, nor earliest sunset of the year,
fall on that day. Hi, I'm Bryan Yeaton for The Weather Notebook.
Instead, at mid-latitudes, earliest sunset occurs in early December, latest sunrise in early
January. These off-sets result from annual variation in the sun's declination, its angle
relative to the equator, and factors related to Earth's rotation and orbital movement relative
to the sun.
To account for these effects, time-keepers adjust sunrise and sunset times using the Equation
of Time. The Equation has two components: one accounts for the day-to-day changes in the
inclination of Earth's equatorial plane to its orbital plane around the Sun; the second
accounts for Earth's orbit being elliptical and not circular.
These factors make the day longer than 24 hours from mid-November to early February, reaching
about 30 extra seconds in late-December. From early December to early January, the Equation of
Time has greater impacts on sunrise and sunset times than changes in the solar
declination.
In the weeks prior to the Winter Solstice, the Equation of Time and declination work in
opposite directions on sunset time. The declination pulls sunset earlier while the Equation of
Time pushes it later. By December 8, the Equation of Time becomes dominant, and sunsets start
occurring later. Meanwhile, both effects are pushing sunrise later and later.
Post-solstice, the situation reverses; both effects now combine to move sunset later in the
day. However, while the declination now works to pull sunrise earlier, the Equation of Time
continues to push it later. This situation prevails until about January 5, then declination
gains dominance, and sunrises begin to occur earlier.
Our program is funded by Subaru of America and The National Science Foundation.
Today's Links
Royal Observatory Greenwich
http://www.rog.nmm.ac.uk/leaflets/equation/equation.html
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