The Story Of Calendar Months!

Why a year has 12 months..? Who named the months and why does the year begin with January..? Why days in months are not fixed and why February is the shortest month..? Why we have leap year and how it can be related to April fools day..?

We know how deeply calendars are embedded in our everyday lives.  Astronomy still lives with us every day in our calendar. As we can see, major sub divisions of time in a year are strictly astronomical. The day is the rotation period of the earth. The year is the orbital period of the earth around the sun and the month is a lunar cycle, or the orbital period of the moon around the earth. Lets look at the origin of our modern calendar of the year and just see what a crazy historical construction it was going back to Roman times.


Naming the months:

Our calendar traces back to the ancient Romans(700BC). They needed a calendar but it didn’t have to be a very good calendar. So the ancient Romans started their year in what we would call March, when the snows melted enough that they could raise an army and go off and do battle. Because they started their year in March and they were following roughly a lunar calendar, they only needed 10 months in their year before they reset it again and essentially they didn’t count the time in the dark winter. So, they named first 4 months of the year after the primordial gods(Martius, Aprilus, Maius, and Junius) and later 6 months with roman numbers for 5,6,7,8,9 &10 i.e Quintils, Sextilus, Septum, Octo, Nove, and Decem respectively. So this was a 10 month’s lunar calendar, excluding 2 dark winter months.


Going Solar:

You might have wondered growing up why are the last four months of our year named after the number seven, eight, nine, and ten when they are clearly the ninth, tenth, eleventh, and twelfth months of the year and the answer comes from ancient Rome. The initial 10 months calendar was followed for few decades and then Priscus and other Roman emperors decided to pad it out into a legitimate lunar calendar by adding a couple of light months at the front end giving us the 12 months. They named these first 2 months of the year after the primordial gods Janus for January(the God of Doorways and Beginnings) and Februa for February(the God of Purity). The year they were matching with their lunar calendar consisted of 354 days. That is 12 lunar cycles each with 29.5 days. So this was a 12 month’s lunar calendar. However, this was not a very good calendar. Because, the Romans with a growing and powerful society, and a need for feeding their growing number of citizens, wanted a solar calendar to plant crops reliably and the existing lunar calendar was no longer working for them. The solar calendar, the time it takes the Earth to go around one complete orbit around the sun, is about 365 and a quarter days. So that’s 11 and a bit days longer than a lunar calendar. In other words, a lunar calendar is no good for agriculture because it goes out of sync by about 11 days a year. So they started to match to a solar cycle. So, the calendar moves towards 365 days. They still had a pretty strictly alternating sequence of 30 and 31 days amongst the months.


The Leap year concept:

640px-Month_-_Knuckles_(en).svg

This solar calendar sufficed for few centuries. But, later rationalization of the Roman solar calendar fell to Julius Caesar, around the time of Christ. The Romans had moved to Sun worship, but Christianity was also becoming a part of their culture. Julius Caesar presided as one of the most extensive and powerful regimes ever known in the world history. It stretched all the way from Africa to Scotland. Regulating an empire this large required messages to be passed around thousands of miles, communications, synchronization of military movements, and the movement of goods and services. It could not be done without a good calendar. So Caesar recognized that the old Roman calendar was not up for the job. And so we have the Julian calendar, the first revolution in calendars where Caesar adds a leap year, that is, an extra day every fourth year. Giving a calendar with a mean length of 365.25 days, and that’s close to a solar cycle. So this was a pretty decent calendar. Caesar had a fairly large ego, he also decided to take the first unnamed numbered month (i.e Quintils) and name it after himself, hence July. Coming soon after Julius Caesar, Augustus his successor didn’t make any innovations to the calendar, but he had a similarly large ego. So he named the next numbered month(i.e 6th Sextilus) after himself, hence August. But he also made the disconcerting reference to the fact that Julius Caesar’s month was longer than his month. So he added a day to his month, hence messing up the pattern. And if you wonder why you have to use the knuckles of your hand to figure out which months have 30 and 31 days is because of Julius Caesar and Augustus and their big egos. So, now we have the idea that the calendar we use is a mash up of ancient and imperial Roman practices and somewhat arbitrary things like the bad luck of February being the last and lightest month.


The Modern calendar:

The Julian calendar is quite successful. However, the exact time that it takes for the earth to go around the sun is one hundredth of a day different from 365.25 It’s actually 365.24199. That difference is trivial, and from one year to the next is truly trivial. But of course, it adds up. So the Julian’s calendar started to get out of synch by approximately a hundredth of a day per year. After 4 centuries that will add up to 3 days(which is why the leap year is skipped in century years which are not divisible by 400) and by Medieval times the Julian calendar was off by a week or so, and agricultural planting cycles were starting to be affected. And so in the era of the great Popes of the Italian Renaissance, we have the Gregorian calendar, the last adjustment made to the modern calendar by Pope Gregory XIII in 1582. Pope Gregory recognized the problem with the calendar, he had court astronomers making careful measurements of the length of the day in a year and so he made a final adjustment, which is built into out calendars that tweaks the average length of the calendar to very close to the true time it takes the Earth to go around the Sun. The Gregorian calendar is basically only off by less than a ten thousandth of a day per year, and therefore will be good for tens of millennia. This is the calendar we live with.

Conversion from Julian to Gregorian dates.
Gregorian rangeJulian rangeDifference
From 15 October 1582
to 28 February 1700
From 5 October 1582
to 18 February 1700
10 days
From 1 March 1700
to 28 February 1800
From 19 February 1700
to 17 February 1800
11 days
From 1 March 1800
to 28 February 1900
From 18 February 1800
to 16 February 1900
12 days
From 1 March 1900
to 28 February 2100
From 17 February 1900
to 15 February 2100
13 days
From 1 March 2100
to 28 February 2200
From 16 February 2100
to 14 February 2200
14 days

The Fools Day:

An interesting side-light on this, is that the protestant countries after the reformation, did not want to follow the Pope’s guidance on the calendar. And so they resisted making the adjustment that Gregory made for another 150 or 200 years. In France, this happened quickly. But in England and the United States, where protestants were dominant, the calendar was not adjusted until the gap between the solar year and the calendar year had approached 11 days. Finally the day, April 4th turned into April 15th. Benjamin Franklin wrote in Poor Richard’s Almanac, widely read by Americans at the time, do not fret over the loss of those days, but people did fret. People worried they’d be ripped off rent, and all bad things would happen! Although the Earth continued in it’s orbit. The French supposedly created the April Fool holiday to mock the Americans and the British for taking so long to get the right calendar.

Citations: teachastronomy.com, Wikipedia

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