Happy Quadrennial Birthday

I am sure most of you will have noticed today is a special day. Today it is the 29th February, a date which only occurs every four years (give or take), and which is a special birthday for those born on the 29th.

While I am sure most people born on the 29th celebrate their birthday each year on either the 28th Feb or the 1st Mar (or why not both), it is of course a special time when their actually birthday, the 29th Feb, rolls around. So for those of you celebrating today, Happy Birthday! I hope you are having a great day, and all the best for the next four years.

Adding a leap day into our calendar is forced upon us because the universe is not perfect. In a perfect universe (dare I say designed?), the Earth would rotate a whole number of days in the time it takes it to complete one orbit of the sun, a year. If things were really perfect, this would be 360 days, as this can then be divided nicely into 12 months of 30days each, but of course we are not so lucky.

Currently (I say currently as over time the length of the day is increasing, but not enough that we need to worry about it here), it takes approximately 365.242 Earth days to orbit the Sun, and you can probably already see the problem, a Solar year is not nicely divided up by Earth days.

To counteract this, Calendars over human history have had to account for the fact Earth days and the Solar year do not nicely divided. This has included additional days, weeks and even a whole month in an attempt to bring the days of our calendar back into line with the Solar year and importantly the seasons. We could of course choose not to correct our calendar, but over time the seasons of the year would begin to drift so they no longer aligned with the months which are familiar to us, and for many this would be a trifle confusing.

The calendar most of us currently use is the Gregorian calendar. Developed in the 16th Century and introduced by Pope Gregory XIII (after who it is named), the Gregorian calendar is an attempt to more accurately track the days and years than its predecessor the Julian calendar.

The Julian calendar had been around for nearly one thousand years by the time the new calendar was introduced, so you might be asking why it needed replacing in the first place. The problem arose in the estimation of the number of Earth days in the Solar year. The Julian calendar used exactly 365.25 Earth years, and accounted for the additional quarter of a day by adding a leap day every fourth year. At this point, you might be thinking ‘isn’t that what we do now?’ but the Gregorian calendar is a little more nuanced than that, as we will see below.

You might have noticed the problem with the Julian calendar. The Solar year is not exactly 365.25 days, in fact it is slightly less, approximately 365.242 days, and although that different is small, over say a millennium the differences begin to add up, so much so that by 1582 when the calendar was introduced, the Julian calendar had already drifted by 10 days. In the year 1582, in the parts of the world which adopted the new calendar, the 4th October was followed by the 15th October to correct this drift. Because the calendar was proposed by a Catholic Pope, at a time when religious divides were, shall we say, tense, meaning those countries which were opposed to the Catholic Church did not adopt the new calendar for decades or even centuries later.

To prevent the drift occurring again, or at least to make a calendar which is more accurate over a longer period of time, the idea of a leap year ever four years needed revising. The idea was close, but not quite close enough. In the Gregorian calendar, leap years occur on any year divisible by four (2020 for example) but not years divisible by 100 (so 2100 will not be a leap year), unless the year is also divisible by 400 (so the year 2000 was a leap year). Does that make sense?

It might sound complicated, but what it actually boils down to is every fourth year is a leap year except approximately once in one hundred years when it is not. Given that most of us will not live to be one hundred, it’s probably not worth worrying about too much.

This post began as a cheerful happy birthday to all the leap babies out there, before descending somehow into a depressing comment on human mortality. I think then we have reached a good point to stop writing. Happy Birthday to all of you celebrating today, and for everyone else, enjoy delaying the arrival of spring by one more day. Leap days don’t happen very often, and since Pope Gregory in the 16th Century they happen even less.

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