A Leap of Calculation
There is something about calendars and dates that is inspiring and almost mystical. The charting of the unknown future in known terms, named and quantified. It is perhaps this ability not simply to mark festivals and celebrations but also to give number to the passing of time that captures the imagination and, for some people, anchors that imagination into some kind of private version of reality — whether it is the illogic of astrology, the lachesism of apocalyptic thinking or the arbitrary numerology of paraskevidekatriaphobia.
I am writing this on 29th February 2016, an intercalary day inserted into the calendar to keep the Gregorian calendar in sync with the seasons and the Sun.
The Moon’s orbital period and period of rotation are the same because they are in orbital resonance, which keeps its numbers simple. The Earth’s day, however, is not in resonance with its orbit around the Sun, so a solar year is a little more than a conveniently precise 365 solar days. For my expected lifetime, at least, leap days are inserted once every four years to compensate for this.
There is nothing extraordinary about this. The attached image I was forwarded at the start of the month is, however, a different matter:
From Feng Shui to pyramid schemes, this is something of a bumper crop of irrationality and suckerbait, easily digested and eagerly passed on. Human culture is awash with half-truths (and lesser fractions) even without the amplifier of the internet, so it can be hard sometimes to sort fact from factoid when it appears in your inbox or via social media. It can be easier to read and accept, based on personal trust, rather than read and check. You could spend your whole life just fact-checking — e.g., with Snopes — but, as your calendar will tell you, time moves on.
In many cases, however, everything you need to know is already in front of you. This is the joy of numbers.
This February won’t come in our lifetime again. Because this February has 4 Sundays, 4 Mondays, 4 Tuesdays, 4 Wednesdays, 4 Thursdays, 4 Fridays and 4 Saturdays. This only happens once every 823 years.
In other words, each of the days of the week, of which there are seven, will occur four times: 7 × 4 = 28. This is the number of days February has in a common year. Rather than once in a lifetime, this situation applies to ~75% of the Februaries in anyone’s lifetime. It is also the one thing that is not true of February this year.
Even without invoking superstition, it can be easy to take in numbers, taking a writer or sender on trust. It is all too easy to slip into a habit of prioritising the words of a text over its numbers. But most of the numbers tossed around in public discourse and the internet do not require advanced mathematics to interpret and negotiate. Most are simple and have simple, calculator-friendly or doodleable relationships to one another. What they require is just a moment to pause and reflect.
On a closing note, even reliable sources can get their facts muddled. Today, for example, Dictionary.com’s word of the day is bissextus. It offers the following definition:
The extra day added to the Julian calendar every fourth year (except those evenly divisible by 400) to compensate for the approximately six hours a year by which the common year of 365 days falls short of the solar year.
The Romans used to insert their intercalary day after the 24th February. It later moved to the end of February. In the Julian calendar system (named for Julius Caesar) the determination of a leap year is simple: a year is a leap year if it is wholly divisible by 4. That’s it. There’s no 400 involved.
These days, however, we use the Gregorian calendar (named for Pope Gregory XIII), for which the leap year calculation is a little more involved: a year is a leap year if it is wholly divisible by 4, with the exception of years that are divisible by 100, unless those years are also divisible by 400. In other words, 2016 is a leap year, 2100 won’t be, but 2000 was.
The definition offered by Dictionary.com is, therefore, incorrect with respect to both the Julian calendar and the Gregorian calendar. Unfortunately, for any verbivores tuning in for the bissextus, it may also be their go-to source of fact for leap year logic.
