Today is the Winter Solstice in the Northern Hemisphere. To be more accurate, it was at 5.30 this morning, London time. That’s the point when the sun reached the Tropic of Capricorn, its furthest point away from us. Today is therefore the shortest day.
I’ve witnessed some animated debate and even a few arguments about this, both on the interweb and in the real world, over the past few days. We are taught that the shortest day and the Winter Solstice occur on 21 December. The trouble is, sometimes they don’t. The exact time of the solstice varies slightly from year to year and therefore so does the shortest day.
To complicate things further, the Winter Solstice is neither the latest sunrise nor the earliest sunset. The evenings start to get lighter by a few seconds from around 16 December. The mornings continue to get darker until 5 January.
Solstice is derived from Latin sol (sun) and sistere (to stand still) because the Sun appears to stand still before changing direction. At this time of year, the rate of change in the length of the day is barely noticeable. It speeds up the closer we get to the equinoxes and slows down again as we near the solstices. Tomorrow, the day will be only five seconds longer. By mid-March it will be lengthening by nearly four minutes a day. Do you ever get that feeling around the August Bank Holiday that the nights seem to be drawing in at a gallop? That’s because they do. It takes two weeks in July for sunset to move from 9:15pm to 9pm. It only takes a week in August for it to move from 8:15pm to 8pm. The position of the Sun is like a train which starts its journey slowly, speeds up in the middle then slows down again towards the end. Of course, the Sun doesn’t actually move, the Earth does but our forebears who first observed the solstices didn’t know that.
These charts (follow the month name links) also show that solar noon varies. Again, we are taught at school that the Sun is at its highest point in the South at 12 noon but it actually shifts by around fifteen minutes either side, depending on the time of year.
Solstices, equinoxes, shortest days, earliest sunrises the timings of noon, then, vary from year to year. The reason for this is that the calendars and clocks we have created for our administrative convenience only approximate to the movement of the Earth. It’s a pretty good approximation and one that has enough precision for our needs. But the Earth does awkward things like orbiting the sun in 365.2563 days, so our calendars can never be exact. We have clever people who add the odd leap day or leap second here and there to keep our clocks in synch with the Earth’s movement. Natural phenomena like solstices and shortest days are, therefore, approximate according to our clock and don’t occur at the same times each year.
Some people seem to find this information disturbing, hence all the arguments on Twitter and elsewhere. Having believed for years that the Winter Solstice and shortest day, latest sunrise and earliest sunset are always on the 21st, folk get quite irritable when they find that they aren’t. Discovering that the seasons are messier and less beautifully symmetric than they had thought annoys people.
We all like to think that there is some order in the world. That’s why religion is comforting and why some people believe in conspiracy theories. It’s preferable to believe that someone, even an evil someone, sees the big picture and is pulling all the strings, than to acknowledge that much of what happens in the world is messy and random. The idea of nice precise seasons gives the same sort of comfort – like looking at something perfectly designed. Only our world isn’t like that. The seasons are, like everything else, messy, approximate and sometimes seeming to lack rhyme or reason. We humans may want precision and order but the world doesn’t always give it. It ignores us and does what it wants anyway.