History + Astronomy = Calendars!

Hey Thinkers!

Here's a brief history I wrote about where our modern western calendar system comes from. It's fascinating to me that this calendar which shapes our lives in such deep ways is rooted so deeply in nature--the earth's revolution around the sun and about its own axis, the moon's orbit around earth and even the planets (think about the days of the week; Monday = Moonday, Tuesday--Martes in Spanish = Mars, Miercoles = Mercury; Use your Spanish or other Romance language to figure out the rest ;)--yet also contains many aspects arbitrarily chosen by humans. I wonder what else follows this kind of pattern...

So, in western calendar schemes, I believe it all started (as so many things around here seem to) with the early Romans (though they were influenced by others, like the Greeks). The early Romans used a calendar system based on the phases of the moon. It takes 29.5 days for the moon to go through one full phase cycle (ie from full to full). So the Romans had 12 months of 29.5 days each (interesting that they'd have half a day in one month and the other half in another...), which comes to 354 days (12 x 29.5 = 354). But they knew that the earth revolved around the sun in 365 days, so they were 11 days short. Therefore, every three years they'd have a 13th month of 33 days to keep the months consistent with the earth's real seasons.

Then, when Julius Caesar took power (~50 BC), he modified the Roman Calendar so that they didn't have to have this extra month every three years. The way he did this was to make the months a little longer than the true time it takes the moon to go through a cycle of phases. So, instead of 29.5 day months, he made them 30 and 31 days, in such a way that with 12 months, you got 365 days (instead of the 354 days of the previous calendar system). If you think about it, you can almost do this by having 6 months of 30 days and 6 months of 31 days: 6 x 30 = 180, 6 x 31 = 186; 180 + 186 = 366--one day too many! So, instead they chose a system in which there are 7 months of 31, 4 months of 30 and one month of 28: 7 x 31 = 217, 4 x 30 = 120; 217 + 120 + 28 = 365! (There are many ways you could do it, but this happens to be what Julius Caesar picked!) Also, because a year is actually closer to 365.25 days, every four years we make that one 28 day month 29 days to keep the months aligned with the true seasons... The months of July and August are named after Julius and Augustus Caesar (They named them after themselves--the hubris!)

I'll also add that in the 16th century, Pope Gregory XIII made some further refinements to the Julian Calendar for two good reasons: One is that a year is not perfectly 365.25 days, but rather ~365.2422 days (though even this length can change slightly over time due to irregularities in the earth's orbit around the sun--caused by gravitational interactions with other bodies in the solar system--requiring us to use leap seconds every so often!), so the Pope made a new rule that we don't do a leap year once per century, unless the century is divisible by 400! All of this is also known as too much information...). The other major reason Pope Gregory XIII modified the Julian calendar is that the earth's spin axis precesses with a period of about 26,000 years. This means that the seasons gradually drift to later dates if you don't adjust the calendar. The last time the calendar had been adjusted was over 1600 years prior when Caesar, in 46BC, changed the calendar for one year to have 445 days--aptly known as the "year of confusion"--Can you imagine how confused everyone would be if all of a sudden, for just one year, we added 80 days?! Anyway, by Pope Gregory's time 1600 years after Caesar's confusingly long year, the seasons were starting 10 days later than they were supposed to. He fixed this in 1582 by dropping 10 days from October that year (thankfully neither Halloween nor my birthday had been invented yet, lest all hell break loose). ANYhow, long story short, because Pope Gregory was the last one to change the calendar up, we now call our calendar scheme the "Gregorian" calendar. Bet you've heard that term but didn't know where it came from till now, eh? You're welcome :)