Calendar Trivia, Part I

I love time zones and calendars.  I'm reading up on calendars today to re-familiarize myself with the different kinds of calendars different populations use.  Mostly for now I'm interested in:

• How Julian Dates work--they're used in astronomy and I used to understand them, but the details have grown hazy
• How a lunar calendar works, because I have no idea, and I have a friend born in Korea who explained that she has two birthdays to sync up with both that calendar and the Western Gregorian calendar, and I was curious to understand this better
  

I'm reading through Dershowitz and Reingold's Calendrical Calculations and some related papers and websites for this.  I'm learning that there are numerous calendaring systems, and I'm not going to read up on all of them.  They all have different epochs (Day 1s) and different ways to measure years, days, and months.  Some calendars start the day at midnight, some at noon, some at sunrise or sunset.

Julian Date is not the same thing as the Julian Calendar.  It's a pretty simple arithmetic system used by astronomers to get rid of confusion about different calendars.  Every day is Julian Day (JD) [some number], and it increases by one every day.  Today (July 9, 2009) is JD 2455022, which started at noon UT (8am EDT).  Days change at noon because astronomers work at night, right?  Tomorrow will be JD 2455023, and so on.  The Julian Period is 7980 years, after which it starts back at zero.  This will happen on January 1, 3268.  Mark your (Gregorian) calendars.

Julian Day 1 was November 24, 4713 BC.  Background on this in Reese, Everett, and Craun, American Journal of Physics 49, p 658-661, 1981.  (The Wikipedia article on Julian Day is also good and cites its sources. Yay!)  I read a bit of this and it refers to several other sources, so I didn't get the complete picture.  Essentialy, the guy who created the system, Joseph Justus Scalinger (in 1583) used a combination of solar, lunar, and Roman tax collection cycles to arrive at 7980. 

Anyway, astronomers like Julian Dates because it's simple to determine when something is happening relative to some other event.  Astronomers sometimes start with January 1, 1980, which was JD 2444240 and add or subtract from there, (mostly, I think, because their software does), as a benchmark.  So, you can say that something that happened on JD 2443240 happened 1000 days ago.  A lot of what happens in astronomy has no relationship to the length of an Earth year, month, or day, so why get bogged down by it?

Unrelated bits of trivia I thought were neat:

• There are two major kinds of calendars: arithmetical and astronomical.  Arithmetical calendars start from some epoch and proceed by a mathematical formula, plus or minus adjustments.  Astronomical calendars are controlled by "irregular astronomical events", with some additional algebraic razzmatazz.
• The Western world uses the Gregorian calendar, named after Pope Gregory XIII, adapted from the Julian Calendar.  It was pretty much the same, but the Gregorian calendar has more complicated leap year rules.  Any year divisible by four gets a leap day, except for century years (years divisible by 100), unless they're also divisible by 400.  2000 was a leap year, 2100 won't be.
• The Ides of a month comes from the Julian Calendar.  I didn't know it was not always in the 15th.  Actually it's usually the 13th.  If it's March, May, July, or October, then it's the 15th.  It also has the Kalends (the 1st of the month), and the Nones (the 5th, except in March-May-July-October, when it's the 7th).  I have no idea what's special about those four months.

Posted at 11:39AM Jul 09, 2009 by WILSON, JOSHUA in Thought experiments  |