Moon Rhythms 1: Why is a Month?

I committed a small paperwork error a couple of months ago.  Having neither looked at a paper calendar nor run through that old rhyme (you know, “Thirty days hath September…”), I scheduled a trip into the woods for a day when I should have been filling out timesheets.  No big deal.  But it did spur the question of why the months vary when the original astronomical timer – the lunar cycle – does not.

I quickly realized that the history of calendars is a rabbit hole I don’t wish to go too far down.  A couple of links led me to more queries, and more links.  Just going to the Wikipedia article on calendars showed a slew of hyperlinks waiting to keep me up until dawn.  But I’m not writing a book on the subject, so if I can provide enough information to whet your thirst for knowledge, that will suffice.

The lunar month, when measured from new moon to new moon, is around 29.5 days long (there are at least 4 other ways to measure a lunar month, but I think this is probably the most relevant for most people).  The cycle of moon phases as it waxes and wanes in the sky is both consistent and spans a useful length of time, making the cycle a natural point of reference for measuring time.  Not surprisingly, the word for “moon” and “month” are closely related or even identical in a number of language groups; in English, “month” derives from Old English monað, which is cognate to mōna, “moon.”

A lunar-based calendar has the benefit of being applicable to anybody with eyes—you see where you are in a cycle and know how long until you reach a different moon phase.  When measuring longer spans such as seasons or years, however, there are disparities.

The solar year…okay, there are a couple of different ways to measure this, too. You will likely be familiar with the Gregorian calendar – the modern western standard – which uses the tropical solar calendar.  By this measure, the solar year is roughly 365.24 days long.  Twelve lunar months fit within a solar year with 11 days to spare.  The discrepancy of a few days per year adds up, so that after a decade, a given lunar cycle in a 12-month year has crept into another season.  The lunar cycles and solar cycles take some 33 years to match up.

Natural cycles don’t care about conforming to our need for whole numbers. 

People across the world have developed different ways of organizing a year.  Going beyond the lunar calendar, we’ve developed several versions of a lunisolar calendar, which marries the lunar and solar timekeeping systems.  Months still follow the lunar cycle.  Generally speaking, some years have 12 months and others have 13. Determining when to apply these “leap months” requires some calculation to determine and a certain level of education to understand.   

Finally, the solar calendar follows the sun’s apparent cycle through the heavens with no concern for lunar position.  There are several, but the aforementioned Gregorian calendar should be most familiar to you. It still uses months, and the months are similar in length to a lunar month, but lunar cycles drift through our Gregorian months with no real connection.  Well, with one exception: Christian churches overlay a lunar algorithm to determine the dates of certain movable feasts – Easter chief among them.

 There are probably over a dozen calendars – solar, lunar, and lunisolar — in use today, and many, many more during the course of human history.  But I’d wager that the basis for each of them lay in one or both of our most prominent celestial bodies: the moon and the sun.