February may only have 28 days this year but while everyone thinks they know about leap years, the rules governing when an extra day occurs are a little more complicated than they first seem...
Why is it called a leap year?
Fixed dates in the calendar, such as Christmas, normally move forward one day in the week each year. For example, 25 December was a Thursday in 2014 and a Friday in 2015. However, in 2016, the last leap year, the extra day in February meant Christmas Day “lept” over Saturday to fall on Sunday.
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A handy way to remember when a leap year is is that they correspond to US presidential election years, meaning the next one takes place in 2020.
Why do we need a leap year?
The time it takes for the Earth to complete one orbit around the Sun is approximately 365 days – but not exactly. One “solar year” actually comes to 365.2422 days. If this discrepancy were allowed to continue, the months would gradually slip out of sync with the seasons so an extra day is added to every fourth year to put things right.
However, one additional day every four years is actually slightly too much, given that a solar year is not a round 365.25 days. To account for this, years divisible by 100, such as 1800 and 1900, are not normally leap years, even though they are also divisible by four. To bring the figure even closer to reality, years divisible by 400, such as the year 2000, are leap years.
When were leap years introduced?
Until the time of Julius Caesar, the Roman Empire followed a 355-day calendar, with an extra month every four years.
However, in 46BC, Caesar introduced the reformed Julian calendar to give the 365-day year, with a leap day every four years. The formula was corrected when the Gregorian calendar was introduced more than 1500 years later by Pope Gregory XIII.
Other date systems still in use, such as the Chinese calendar and the Solar Hijri calendar in Iran, also account for leap years.
What happens if you are born on a leap day?
First off, you're very unusual. The odds of being a leapling, as leap day babies are sometimes called, are one in 1,461.
However, you also face the prospect of spending the majority of your life with no official birthday. For legal purposes, such as voting or purchasing alcohol, people born on leap day are allocated a 1 March birthday on non-leap years.
Leap year traditions
The most famous custom associated with leap years is that of women proposing to men. Folklore has it that St Brigid, an Irish nun, sought and gained permission from St Patrick himself to allow women to propose on leap day.
Scotland began the tradition in 1288 by passing a law permitting women to propose and if refused, the man had to pay a fine, traditionally paid in the form of a new gown, money, or 12 pairs of gloves for the lady.
In many countries, including Denmark and Finland, a refusal was traditionally accompanied by presents of clothing to soften the blow.
“The thinking behind the gloves was that the woman could then wear them to hide her embarrassment as not having an engagement ring” says the Metro.
However, neither men nor women are likely to be doing any leap year proposing in Greece, where it is considered an unlucky time for marriage. Similar superstitions about leap year's supposed infelicity also exist in Italy, where the saying goes “anno bisesto, anno funesto” (leap year, gloomy year), and Russia, where leap years are associated with freak weather and a higher risk of dying.
If you are born on a leap day you're normally invited to join The Honor Society of Leap Year Day Babies, although Scots now consider it unlucky to be born on 29 February.
