As we all well know, the winter solstice, the shortest day of the year is usually a couple of days before Xmas, so you'd expect the sun to rise later and set earlier every day until the solstice; and then to start rising earlier and setting later every day thereafter.
That turns out to be not quite true. According to this fine article, the earliest sunset is much earlier - around 13 December, and the latest sunrise is much later - 6 January. It's just that until the winter solstice, you are losing more in the morning than you gain in the evening; and thereafter you are losing less in the morning that you are gaining in the evening.
We live and learn. I'm sure that'll come in handy in a pub quiz one day.
Monday, 14 December 2009
Those long December evenings ...
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4 comments:
It's called 'precession of the equinoxes' and its the real cause of climate change. Look up 'Milankovitch Cycles'
Anon, thanks for that. It does all look a bit complicated - if you have all thse different cycles with different impacts (which sometimes cancel out and sometimes reinforce) how on earth do you disentangle it all again?
I have been so worried about this anomoly that I just couldn't sleep easily. I am so relieved that you've cleared it up. Phew!
"The effects of the elliptical orbit and tilt of the axis are particularly dominant at this time of the year and cause the time at which the sun crosses the meridian to change by ten minutes between the 16th December (the sun is highest in the sky at 11:59am) and 5th January (the sun is highest at 12:06pm)."
12:06 - 11:59 = 10 mins?
Is this the maths they teach in school these days?
The rest of the explanation is crap too.
The key fact is that one day is not one rotation of the Earth, but one rotation plus the angular advance of the Earth's orbit around the sun, i.e. 360 + approx (360/365.25).
Because the orbit is elliptical, the Earth does not advance around the sun the same amount every day, but goes faster when it's closer to the sun and slower when it's further out. This means that days, measured from noon to noon, are not always exactly 24 hours, but average out over the year.
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