Ken Tapping, January 1st, 2014
High on the southern wall in the church of Saint Sulpice in Paris, there is a hole. At local noon, the sun shines through this hole and projects a spot of sunlight on a bronze strip that runs north-south across the floor. As the sun cycles from being low in the sky at the winter solstice to high in the sky at the summer solstice, that spot of light falls on different parts of the strip. In the winter the sun is so low that the church is not wide enough for the spot of light to fall on the floor, so at its north end, the strip turns into an obelisk, Thanks to the popular movie, “The Da Vinci Code”, there is a newly made sign on the obelisk, saying that there is absolutely nothing buried under the obelisk.
This bronze strip, which defines a north-south meridian, was set up in 1727 so that the spot of sunlight would tell the local clergy the date, in particular when to celebrate Easter. It underlines that even as recently as the 18th Century, knowing the precise date could be difficult. Having your own meridian enables you to ignore the calendar.
Our remote ancestors needed to know when to sow or reap, and when the fish, birds and animals they depended upon for food would be available. Missing out could be fatal. Our astronomical ancestors needed to know dates and times, but the need to know the date impressed itself on most of us when organized religion came along, with its special festivals and celebrations. In the case of the Christian church, knowing when to celebrate Easter and Christmas is critically important.
Counting days as the Moon goes through its phases is a start. The word "month" comes from "moon". The interval between two consecutive Full Moons is 29.53059 days. That's a problem; the lunar month is not a whole number of days, or even half days. So over several lunar months, the day count drifts and its hard to tell exactly what the date is. Even worse, a year, the time between two consecutive spring equinoxes, which is also the time taken for the Earth to complete one trip around the Sun, is neither a whole number of days (365.24219) , or a whole number of lunar months (12.368249...). The result is that any convenient day counting calendar accumulates errors. A lunar calendar drifts unless carefully managed.
When he was Dictator in Rome, Julius Caesar decided the calendar in use at the time was a mess, and decreed that a system was needed that stayed synchronized with the annual motion of the Sun without a continuous need for correcting it. In this reform a 365.25 day year was used and the winter solstice was locked to 25 December. This calendar is now known as the Julian Calendar. When the Christian religion spread across the Roman Empire, the solstice celebrations were rededicated. This worked for a while, but since the year is not 365.25 days errors started to accumulate. In 1582 the situation had become unacceptable and Pope Gregory corrected it again, he adjusted the date to get Easter back to the right time and came up with the correction formula do deal with the remaining errors that we still use: "If a year is divisible by 4 but not exactly by 100, unless the century is divisible by 400, that year is a leap year and we assign it 366 days, otherwise it is 365 days. That works a lot better, but there are still small errors that accumulate. To correct those as needed, we add a "Leap Second" to keep the date always accurate to within a second. By definition, Christmas stayed on 25th December, but the solstice got moved to the 21st.
The Gregorian Calendar is now widely used, but by no means universally. For example, the Greek Orthodox Church adheres to the Julian Calendar, which, according to the Gregorian Calendar, celebrates Christmas on 7th January.
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