Leap Year: Climate meets Astronomy meets Society

February 29 – a day that comes around only once every four years, except for years divisible by 100, unless that year is also divisible by 400. Most people don’t give these complicated rules much thought; our calendars and computers keep track of it for us, and the addition of a leap day every four years is little more than a passing curiosity. The reasons behind leap day, and how it came to be, though, are anything but trivial. This month we’re going to take a closer look at those reasons, so get ready for a foray into history, timekeeping, astronomy, climate, religion, and society.

So, why even bother with the hassle of adding an extra day every fourth year, except in years divisible by 100 that are not also divisible by 400? The key is in the ratio of the earth’s rotational period (day length), and the time required to complete one revolution of the sun (solar year length). Namely, the two will not divide evenly into one another—one solar year is 365.2424 days long. If we simply chose to make our calendar year 365 days long (ignoring the .2424 day remainder), our calendar would slowly drift out of phase with the seasons: the northern hemisphere winter solstice might initially occur in December, but as the calendar accumulated more and more error (at the rate of about a quarter of a day per year), the winter solstice would drift earlier and earlier until traditional “winter” months no longer occurred during a winter climate—within 500 years, the winter solstice would occur in late August! This rate of drift is significant enough that it would be noticeable even within a single human lifetime. Society has generally found this drift to be undesirable, and many different methods have been used throughout history to address it.

To complicate matters further, the earliest Greek and Roman calendars were lunar calendars—that is, calendars consisting of months based on the time required for the moon to orbit the earth. One such Roman calendar, used in the 7th century BCE, consisted of 12 lunar months of 28 or 29 days each, for a total of 355 days. To account for calendar drift, a thirteenth “leap month” was added roughly every other year. This calendar system was later replaced by the Julian calendar, introduced by Julius Caesar in 46 BCE. The Julian calendar abolished the use of leap months, and instead introduced a system that tied the length of the calendar year to that of the solar year. The Julian calendar was identical to the modern calendar except in one regard—leap days would occur every fourth year, providing an average calendar year length of 365.25 days, close enough to the solar year length to greatly slow the observed seasonal drift. Under the Julian calendar, after 500 years, the calendar would only have drifted away from the seasons by about 4 days.

Even this was not good enough for some, however. Our modern calendar system, the Gregorian calendar, was introduced in 1582 by Pope Gregory VIII to prevent the observance of Easter from drifting with regards to the seasons. To improve upon the Julian calendar, the Gregorian calendar added the rules regarding years divisible by 100 and 400, resulting in an average calendar year length of 365.2425 days, extremely close to the true solar year length—in fact, after 500 years using the Gregorian calendar, only about 3 and a half hours of seasonal drift would be observed. Other calendar systems use still different strategies to prevent seasonal drift—the traditional Chinese calendar, for example, uses a ‘lunisolar’ system, where the year length is defined by the solar year, but months are numbered according to the phases of the moon. Even today this traditional calendar is still used to determine the dates of holidays such as Chinese New Year or the Mid-Autumn Festival.

What future changes we’ll see to the calendar system remain to be seen—but for now, enjoy your leap day!





Story is © Nate Snook, 2008