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Mighty Rome decided that the system in place was just to difficult to manage and the Romans needed something that would become world wide in use, so they set to work make a calendar that would be used throughout their whole empire.
This wasn’t as easy as one would think. The Romans complicated their lives by being so superstitious- even numbers were unlucky. Therefore the months were either 29 or 31 days., but- when these days were added up there were only 355 days. Not to be fazed by this, they simply added another month calling Mecedonius containing 22 or 23 days; and it was tacked on every second year. However, their calendar eventually became so far out that Julius Caesar, advised by his foremost astronomer Soigenes, ordered a sweeping reform in 46BC where upon it was made 445 days long by decree. This finally bought the calendar back into line with the seasons, then the solar year, with value of 365 days and 6 hours. To take care of the 6 hours, every fourth year was made a 366-day year. For his finale, Caesar decreed the year was to begin with the first of January and not with the vernal equinox – and Caesar, was not one to argue with!
The original Roman year had 10 named months:- Martius = March; Aprills = April; Maius = May; Junius = June; Quintills = July; Sextilis = August; September =September; October = October; November = November; December = December. Possibly also two un-named months in the middle of winter when nothing happened in agriculture.
Of course seeing he had so much to do with it and probably others working on it, to keep on Julius Caesar’s right side, they named it the Julian Calendar after him. All Eastern Orthodox churches use it for calculating holidays to this day. The year began with Martius – March. And it was the second King of Rome, Numa Pompilius circa 700 BC who added the two months Januarius = January and Februarius = February and it was he who ordered the change of the month to Januarius at the same time changing the number of days in several months to be odd – a lucky number. After Februarius there was occasionally an additional month of Intercalaris – intercalendar. It is interesting to note it is still 111/2 minutes longer than the solar year- so after a number of centuries 111/2 still adds up.
Caesar’s calendar survived through to the 15th century and up till then it had about a week making the vernal equinox fall around March 12 instead of the 20th. It was then that Pope Sixtus IV, (who reigned from 1471 to 1484) called the German astronomer Regiomontanus to Rome to advise him (the Pope) on a reform of the Julian Calendar. Regiomontanus arrived in `1475 but he died shortly after his arrival, so the Pope’s plans for a reform died with him.
The Council of Trent in1545 authorised Pope Paul III to reform the calendar once more. A huge amount, in fact most of the mathematical and astronomical work was done by a Father Christopher Clavius, S.J. The Father didn’t waste any time and immediately a correction was made and Pope Gregory XIII decreed Thursday, October 4th 1582, was the last day of the Julian Calendar., the next day was Friday, 15th October. (One wonders how many people pondered over the fact they had lost days! Our days go so fast in these modern times that we don’t lose days – we seem to lose weeks!)
A formula was suggested by the Vatican librarian- Aloysius Giglio was adopted. For long-range accuracy every fourth year is a leap year unless it is a century year like 1700 or 1800. Century years only - divisible by 400 can be leap years - i.e.1600 and 2000. This rule discards three leap years in four centuries thus making the calendar sufficiently accurate.
One has to note though, that in spite of the revised leap year rule, an average calendar year is still about 25 seconds longer than the earth’s orbital period. But believe it or not it will take 3,323 years to build up to a single day! So none of us or even our Gt, Gt, Gt, (+ six times removed) grandchildren will be around to see the calendar have another revision!