## Tuesday 22 December 2015

### Doubting equinox

As Dava Sobel explained in her book Longitude, determining precise details of the position of the stars above the horizon is [only?] of vital importance for navigation at sea. Nobody on land is going to die if their observation of Rigel or Sirius is faulty by a few degrees or a few minutes. At sea, especially nearing land after an oceanic voyage, particularly in dirty weather, you really need to know where you were on your last clear sighting of a known star. Otherwise you are likely to pile up on a Scillian reef like Sir Cloudsley Shovell and hundreds of his sailors did on 23 October 1707. So for astronomical data you defer to United States Naval Observatory (USNO) and/or United Kingdom Hydrographic Office (UKHO) probably even if you are part of La Francophonie. I had two txt-greets yesterday from tree-hugging pals saying "Happy Solstice".  Which I had to say was wrong wrong almost right because the Solstice isn't always on the 21st December, but swings between the 20th and 22nd and this year it falls at 0448hrs UCT today 22nd December 2015.  In the period 1900-2100 the Winter solstice has fallen 5 times on the 20th; 103 times on the 21st; 91 times on the 22nd and in 1903 edged 20 minutes over into the 23rd.  So dear readers, before you make a faux pas over the equinox this coming March check with the US military.  I can just see your pagan gods face-handing upstairs "Jaysus! They've got it wrong again, if our loyalest supporters are going to get their kit off to dance in midwinter could they not get the date right?".

There are two things going on at midwinter that don't match up precisely. It would be a lot more convenient if the Earth rotated on its axis exactly 28x more than the Moon went round the Earth. Actually it would be better still if the ratio was 30x and the Earth's journey round the Sun took exactly 12 of these integer moonths. But Zeus and Loki and Ganesh have arranged it as a mathematical challenge. The solstice is about the obliquity of the axis [N pole to S pole] about which the Earth turns w.r.t. the plane about which the Earth orbits the Sun. This angle is 23.4 degrees and explains [not here though!] why the Tropics of Cancer and Capricorn are 23.4 above and below the Equator. Weirdly and wonderfully the Earth shuffles round on its orbit with the tilt always pointing in the same direction. When the the N pole is pointing as near as it can directly at the Sun, then that's the Solstice. From our Earthbound Northern Hemisphere point of view, the rising Sun creeps closer and closer to a most Southerly point [sun-stand-still prevlier]and then edges back North again.  Stonehenge and Newgrange are aligned to this point on the horizon.

The other thing that is off centre is that Earth's orbit is not precisely circular as required by Plato with his celestial spheres singing their heavenly song. Ptolemy had to fudge the actual data with a raft of secondary circular epicycles to force 'circularly' on the heavens.  Copernicus had a great leap forward when he suggested that planetary and starry prediction would be easier with the Sun at the centre rather than Jerusalem or Alexandria. As Koestler [prevlier] explained in his book Sleepwalkers, there were more epicycles after Copernicus had finished than under the old system, but at least it was a step in the right direction. It took Kepler to twig that planetary orbits were elliptical rather than circular. Earth's orbital eccentricity is about 3% and the perihelion (closest approach) occurs a couple of weeks after the Solstice, as I explained a year ago. The earliest sunset and the latest sunrise and the shortest day are close but not exactly on the same day as the solstice: it's complicated. Obviously, the tilt of the Earth away from the Sun offsets the fact that the Sun is 3% closer but the tilt is much more significant in its effect on our weather. For us in Ireland it has been the most miserable weeping wet December that I can remember.  With my two-week event horizon that's true for every Winter, hmmmm? If the tree-huggers [meeeeee!] would just do their dance today, maybe the weather gods would cut us some slack?