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What gets leapt in a leap year?

2012 is a leap year in the Gregorian calendar, making it an appropriate time to consider the origin of this rather puzzling term. After all, leap implies that something is being skipped over, but a leap year has an extra day, making it longer than an ordinary year, not shorter. Where is the metaphorical leap in a leap year?

Leap year is attested from the 13th century, but probably goes back even earlier, to judge from the Old Norse equivalent hlaup-ár. It is thought that the “leap” refers to the impact of the extra, or intercalary, day upon which day of the week specific dates fall on in the year that follows. In a standard 365-day year, a calendar date which fell on Wednesday in the previous year falls on Thursday. The extra day in a leap year causes calendar dates to move ahead by two weekdays rather than just one. Thus, 2011’s Christmas fell on Sunday, but the Christmas of 2012 will take place on Tuesday, not Monday. It is that ‘leaping over’ a day of the week that gives the leap year, and hence also the leap day, 29 February, its name.

Another “leap” term has also been in the news this year: the leap second, which is added periodically to the atomic reckoning of time to keep it in line with solar time (one is due to be added on 30 June 2012). On 20 January, experts at the International Telecommunication Union, who had been considering abolishing the leap second, spared it (for now, at least). The term leap second is modeled on leap day, but while that phrase has been with us for over 400 years, leap second will be lucky to reach the age of 50. Introduced in 1972, the intercalary second has only been granted a temporary reprieve; the proposal to abolish the leap second will be considered again in 2015.