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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>>
>>
>
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