Oops , sorry -- Hit the wrong reply button on THAT one.
Happy Solstice to all.
Steve W
On Dec 20, 2010, at 11:16 AM, Steve and Ann wrote:
Sheila:
Looks like what we said; and I think it's OK. (One typo: "Betsys"
should be "Betsy's".)
Steve
On Dec 20, 2010, at 9:26 AM, Kevin Karney wrote:
Dick
Thanks for that correction. I am sorry I was being so insular and
did not think it all through.
One is seeing the interplay between two equations :
1) sunset/rise being (more or less) = 12 +/- [acos{tan(latitude) .
tan(declination)}]/15 with declination change rather minimal around
the solstice
2) equation of time changing rapidly around the winter solstice.
There is a third influence - but I think it probably plays an
insignificant part - the difference between the usually quoted
geocentric equation of time and its topocentric cousin - which
varies the equation of time by up to a second dependent on both
time of day and latitude.
Best regards
Kevin
On 20 Dec 2010, at 12:39, kool...@dickkoolish.com wrote:
The date of earliest sunset depends on Latitude and is not
always Dec 14. See: http://www.idialstars.com/eass.htm
Dear Colleagues
In the depths of a freezing Northern winter, there's time to
think about
days of special interest. There are six significant solar dates
around
this time of the year.
Three are special and probably known to all...
21st December - half an hour before midnight - the Winter Soltice
25th December - not only Christmas day, but one of the four days
in the
year when the equation-of-time is zero.
3rd January - Perihelion when the Earth is closest to the Sun,
One day is perhaps of interest only to the serious heliochronometer
user...
23rd December - the day when the equation of time is changing at
its
maximum rate of almost 30 secs/day
Two days are nothing more than obscure, except to the numerically
obsessed
- or to those who like strange questions to ask on quiz nights....
14th December - the day on which the Sun sets earliest in the day
29th December - the day on which the Sun rises latest in the day
See the graph below. If you wonder why this is so, it is because
there are
two effects in play (a) the daily change in sunrise/sunset as a
result of
declination change is minimal around the solstice and (b) the
effect of
the equation of time with its large eccentricity component as the
Sun
races forward towards perihelion.
p.s. the graphs are for 54 degrees North latitude; the y-axis
scale will
change at other latitudes.
p.p.s. dates are UT - so they may vary with your local longitude.
Have a wonderful Christmas season.
Kevin Karney
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