In four years, the delay adds up to 23 hours, 15 minutes and 2.04 seconds, so adding a leap day now makes it start the next year some 45 minutes early. After 128 years, those 32 batches of 45 minutes each will have added up to a full day.
Julius Caesar decreed, in 45BC, that every fourth February should have an extra day in February. Unfortunately, as he forgot the centuries, the world gradually edged ahead of the calendar. Between the Council of Nicaea in AD 325 and an edict of Pope Gregory XIII in 1582, the date of the spring equinox (important for calculating Easter) slipped from 21 March to 11 March. On the advice of his astronomers, Pope Gregory solved the problem by ordaining that 5 October 1582 should be called 15 October, and thenceforth no century year would have a leap day, unless the century was itself divisible by four. So 1600 was a leap year, but 1700, 1800 and 1900 were not. And 2000 will be. Most of Europe adopted the Gregorian calendar in 1582 or 1583. Britain took until 1752 to make the change, by which time it was 11 days adrift and had to lose all the days between 3 and 13 September.
The Gregorian calendar thus operates on a 400-year cycle, with a total of 97 leap days. That gives 146,097 days every 400 years, an average 365.2425 days per year, or 365 days, 5 hours, 49 minutes and 12 seconds.
So we are still slipping by about 27 seconds a year. Which means we may need another leap day adjustment in some 3,000 years' time. Despite the accuracy of the Gregorian calendar, however, one cannot help feeling sorry for anyone born 100 years ago today, who would not have had a true birthday until they were eight.
None of which explains the custom that women may propose marriage on 29 February. That has nothing to do with Julius Caesar or Pope Gregory but is, according to Brewer's Dictionary of Phrase and Fable, the result of delicate negotiation by St Patrick.
He was, Brewer claims, walking along the shores of Lough Neagh, when accosted by a tearful St Bridget. Her nuns, she explained, had mutinied over the right to propose marriage. When St Patrick offered to allow them to do so every seven years, St Bridget threw her arms round his neck and said: "Arrah, Patrick, jewel, I daurn't go back to the girls wid such a proposal. Make it one year in four." St Patrick replied: "Bridget, acushla, squeeze me that way agin, an' I'll give ye leap-year, the longest of the lot."
Bridget then proposed to Patrick, who solved the problem by giving her a kiss and a silk gown instead. Since when, it is said, any woman whose leap year proposal is turned down may claim a silk gown.
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