Did you have to memorize that 16th century nursery rhyme when you were a child?  I did, and it has proven surprisingly helpful to me as an adult.  I did find, however, that you only needed to know the first line, “Thirty days hath September, April, June, and November” to be able to know immediately how many days are in any particular month.  You know if it doesn’t have 30 days, it has 31, except February, of course.  Knowing if it was a leap year came later, because if a year can be evenly divisible by 4, it’s a leap year and February has 29 days.

La mojarra inscription - Mayan calendar

That’s simple stuff, right?  You’ve probably known it all your life… although come to think of it, the old rhymes and tricks aren’t taught anymore.  Seems now there’s a whole generation of people who have to see a calendar to know how many days May contains, while flipping frantically around for February to see if it’s a leap year.  In our technological age, rhymes and tricks like this seem terribly outdated and simplistic.  Still, as smart as we are we must  remember that not everybody is literate, or has access to a computer, calculator, or calendar.  For them, this is magic.  You see, even if you spend your life in a cave, squatting around a campfire, you need some form of calendar.  You need to know when the weather will change; when the fruits will ripen; when the herds will move; when the winter will come.  The easiest way to do this originally was to look at the phases of the Moon; count the days in each lunar cycle, split it down to bite-sized chunks you can remember.  Now you know if you’re using your winter stores too fast, and need to ration your food out so nobody starves to death.  That’s the origins of the lunar calendar, and that’s when knowing the basics of astronomy would save your life.

Stonehenge, Image by Frederic Vincent, some rights reserved

The lunar cycle doesn’t correspond exactly with the solar cycle, as we all know.  There is an approximate 11-day difference there (the solar cycle is the longer), so if you use a lunar calendar every year you would have to tweak your calculations to factor in the solar gain.  While the lunar calendar does survive in modern times as more than a footnote (the Islamic calendar is lunar), almost all modern calendars are either solar/lunar hybrids (the Hebrew calendar), or purely solar calendars (the Gregorian calendar).

In Western culture, you’re probably most likely to use the Gregorian calendar, which is a standardized solar calendar.  It divides the tropical year into regular, predictable blocks of time called months, weeks, and days.  Every four years it adds a day in February.  Without even paying much attention to it, you probably divide your time using three or four different systems; you have a calendar year, a fiscal year, a year dividing religious observances (if practiced), and a school year, just to name four.  As we become more an more familiar with different cultures through the Internet, you’ll find yourself thinking in more and more calendar divisions.

Astronomical Clock, Prague - Image Maros M r a z, some rights reserved

As familiar as we all are with overlaying cultural, religious, and social issues onto a calendar, at rock-bottom it is still all based on the orderly progression of our planet and its moon through the solar system and the galaxy.  As long as we plan on eating, we need to know when the food will be available.  We need to know when it’s going to be cold.  So, what does astronomy have to do with a calendar?

Everything.