Tomorrow December 21st is the winter solstice, the shortest day (longest night) of the year. It is the moment, actually 9:30 pm to be precise, when the sun reaches its most southerly declination of -23.5 degrees. That means that the North Pole is tilted 23.5 degrees away from the sun. At this point, as the sun appears to pause then start its journey northward, it means that, for us, the days start getting longer again and the sun climbs higher in the sky.
Figuring this stuff out comes down to folks at time laboratories calculating nanoseconds of time with the use of atomic clocks in Greenwich, England, the original ground zero of world time, and other time labs around the world. Greenwich Mean Time (GMT) became the world’s time standard at the Washington Meridian Conference in 1884. It was agreed back then that the Prime Meridian (the longitude defined to be 0 degrees) passed right through the Royal Observatory, Greenwich, in southeast London, U.K.
This lofty time point has other more convoluted references. It is also known as Coordinated Universal Time (UTC), it’s sometimes called UT1, and it’s used as Zulu Time in aviation. Getting the time right is set by the International Bureau of Weights and Measures based in Sevres, France and known by its French acronym BIPM – Bureau International des Poids et Mesures.
The BIPM’s Time Department calculates UTC based on input from some 400 atomic clocks held in 70 national metrology institutes, observatories and other institutions in 48 nations. The world-wide distribution of timing centres guarantees accuracy to a nanosecond (a billionth of a second) and broad dissemination of UTC.
BIPM doesn’t stop with time. Under the Metre Convention of 1875 (which created BIPM in the first place in a treaty ratified by 55 nations), the agency ensures the uniformity of units of weights and measures around the world. It’s the official keeper of the international prototype of the kilogram. Who knew?
Ancient people didn’t, of course. But they knew the significance of the winter solstice enough to know that mid winter was a turning point for the lengthening of the day and the returning warmth of the sun. That was as good excuse as any for celebrating, feasting on some freshly slaughtered cattle and downing some well fermented beer.
From Yule traditions in Nordic lands when large logs were burned to the worship of Oden in northern Europe and Saturnalia, Juvenalia and Mithra in Italy, pagan gods were remembered in the time honoured ritual of eating and dancing that went on for days.
December 25th became the birthday of the ‘invincible sun’ in the third century according to the dictates of Emperor Aurelian. Then, in 273 the Christian church set this date as the birth of Christ and by 336 the Roman feast to the sun became the celebration of Christ’s birth.
There was likely a morsel of politics mixed up in this.
By holding Christmas at the same time as some of the traditional festivities, religious leaders theorized that the Christian celebration would become more popular. They were right on that score. Over the centuries, pagan rituals were replaced with church-going, celebrations, feasting and gift-giving. Christmas came to dominate the mid-winter festivals, triggering new customs and traditions along the way.
The exact date of Christ’s birth, of course, isn’t known beyond a guess or two at the year, historians putting it sometime between 7 and 2 BC. And the season could be off. The concept of shepherds tending flocks of sheep in the night maybe puts the date closer to lambing time.
But either way, the winter solstice and Christmas are times of special celebration.