Calendars

The Roman calendar was pretty mucked up in Julius Caesar’s time, and he was getting fed up with it. He realised that the Greeks were the clever ones who could sort it out for him, and asked Cleopatra to do something about it. It helped that Cleopatra was his mistress at the time. So, Cleopatra asked her ‘chief scientific officer’ Sosigenes to sort it out. His calculations produced the Julian calendar, used for the next 1600 years or so. Months of 31 and 30 days alternated, except for poor February, which had a few lopped off. And when Julius was designated a god, he had a month named after him; and likewise his successor Augustus who got the month after Julius — which partly explains why September, the seventh month, is nowadays the ninth month. But, as Augustus couldn’t be seen to have fewer days than Julius, Augustus’s month had to be made up to 31 days. And guess where the extra day came from!

The Roman calendar started at the founding of Rome — the exact date varies with the source. ’Little Dennis’ was given the task of dating the Christian calendar from the year of Jesus’s birth which would have been 1AD — he seems to have got this a few years out. And in the fifth century, when he was doing the sums, the concept of zero or ‘0’ was unknown; so 1BC was immediately followed by 1AD. We call these BCE (before the common era) and CE (common era) these days.

By the end of the sixteenth century, the calendar was in trouble again; it was 10 days out. This was important as it made the calculation of Easter problematical. So Pope Gregory got his scientific chaps to revise and correct the Julian calendar, which they did. They had to shift the date by 10 days, but, in an attempt at perpetuity, altered leap years. Only centuries divisible by 400 were to be leap years, rather than every century — the other leap years remained. Thus entered the Gregorian calendar, the one that your computer uses.

Copernicus published his ‘theory’ that the earth went around the sun earlier in the sixteenth century, though this went against orthodox theology. Nonetheless, Gregory’s scientists used this concept in their calculations — the sums were easier. And by describing the ‘revolution’ of the earth, Copernicus unwittingly added a new meaning to the word — a successful ‘regime change’.

Sosigenes’ error of 10 days in 1600 years is about 1:58,000. Not bad going for someone without the benefit of a telescope, a calculator or a computer.