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
Sunrise-Sunset.pdf
Description: Adobe PDF document
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