Today is February 29th, a day that appears on the calendar once every four years (or close enough). 2012 is a leap year.
The necessity for adding a day to the calendar every four years is due to the fact that the Earth’s orbit of the sun is not exactly 365 days; it’s closer to 365.25 days. Therefore, about every four years we add one day to the calendar to keep the seasons aligned with the calendar. (For an excellent technical discussion--it’s not this simple--see Phil Plait’s Bad Astronomy blog entry on the subject here.)
But we’re concerned with the word here. The use of leap to denote calendrical shifts like this dates to Old English. It appears in AElfric’s De Temporibus Anni, written c. 993. AElfric of Eynsham was a Benedictine monk who is probably the chief prose stylist of the late Old English period. De Temporibus Anni is his attempt to provide monks and priests with a text on astronomy and the calendar that they could use in the education of themselves and the laity and in combating superstition and myth. AElfric wrote in reference to the moon (which needs a leap day added to its orbit of the earth about every 19 years):
se dæg is gehâten Saltus lune • þæt is ðæs monan hlyp
(the day is called Saltus lune, that is the leap of the moon; saltus lunae is Latin for “leap of the moon,” and medieval writers frequently elided the Latin genitive -ae ending into -e)
The leap comes from the idea that the calendar jumps and does not proceed in an orderly fashion.
The term leap year isn’t cited in English until 1387, when it appears in John de Trevisa’s translation of Polychronicon Ranulphi Higden:
That tyme Iulius amended the kalender, and fonde the cause of the lepe yere.
(That time Julius amended the calendar and established the cause of the leap year.)
While the term leap year isn’t recorded until the Middle English period, it probably was in use in Old English. The term hlaup-ár, or leap year, is recorded in Old Norse and most Norse calendrical terms were borrowed from Old English. So it seems likely that Norse acquired this one from Old English too.
(Sources: Oxford English Dictionary, 2nd Edition; Dictionary of Old English Corpus, 2009)
Copyright 1997-2015, by David Wilton