"Andrew May" wrote in message
m...
"Mark" wrote in message
...
Look everyone...we need to clear up one or two points here.
(1) 21st June is the Summer Solstice, which to most people is
traditionally
called Midsummers Day or the 'Start of Summer' (which I think is weird,
but
hey...). It is the longest day of the yesr for the northern hemisphere,
and
is determined by the Earth's orbital inclination, or tilt, of 23.5
degrees.
I've come a little late to this thread so may have missed something -
but there is something about Midsummer?s Day which no-one seems to
have mentioned. At least this is my understanding of the definition.
The longest day or the summer solstice is (by definition) the day with
the longest time between sunrise and sunset. I don't think anyone
would disagree with that although astronomically it is not always on
the 21st June because of drifts around the 400 year cycle of leap
years.
The true year length is 365.256 days, but we use the Tropical Year (which
allows for the precession of the Earth's tilted axis) to keep our dates
constant - this is 365.2422 days...so we have leap years. These keep our
calendar in step, not out of it!!!
1 extra day every 4 years would account for 0.25 days and would make the
calendar out of step by 3 days every 400 years! So we drop the leap day
every 100 years except when it is divisible by 400 - e.g. 1600, 2000.
So, with this system in place, all solstices and equinoxes fall on the same
day each year! It produces an error of 1 day every 300 years!
Although the solstice is the longest day bizarrely it IS NOT the day
on which the sun sets the latest - that occurs about four days later
around the 25th June. Of course the sun rises proportionally later so
the day is shorter than the solstice. I have always understood this
day to be Midsummer?s Day.
Not true - see below!
I guess what I am trying to say is that Midsummer Day celebrations
occur when they do because of an astronomical phenomena not on some
arbitrary date.
If you want to check this for your own location you can get a complete
list of sunrise/sunset times he
http://www.schoolsobservatory.org.uk/uninow/sunrs/ and there is more
about timekeeping and astronomy at the Royal Greenwich Observatory
site he http://www.nmm.ac.uk/site/navId/00500300f00h including a
definition of he Equation of Time which (I think) explains this
difference in terms of the earths tilt and its elliptical orbit around
the sun.
That is a GREAT site - must use that in future. However, if you look at a
variety of longitudes, the latest sunset falls between 21st June and 6th
July - it is not the same in all places. e.g. try pacific coat of the U.S. -
latest sunset is 29/6 to 1/7!!!
So, still has no real meaning!
Just my contribution. Now back to the gardening.
Andrew