The "leap day" of 29 February exists for purely astronomical reasons, but has always prompted less scientific curiosities.
Here are 10 things to consider - for one day only. Until 2016, that is.
1. The leapyear's extra day is necessary because of the "messiness" of our Solar System. One Earth year (a complete orbit around the Sun) does not take an exact number of whole days (one complete spin of the Earth on its axis). In fact, it takes 365.2422 days, give or take.
2. Until JuliusCaesar came to power, people observed a 355-day calendar - with an extra 22-day month every two years. But it was a convoluted solution to the problem and feast days began sliding into different seasons. So Caesar ordered his astronomer, Sosigenes, to simplify things. Sosigenes opted for the 365-day year with an extra day every four years to scoop up the extra hours. This is how the 29 February was born. It was then fine-tuned by Pope Gregory XIII (see below).
3. Every fourthyear is a leap year, as a rule of thumb. But that's not the end of the story. A year that is divisible by 100, but not by 400, is not. So 2000 was a leap year under the Gregorian calendar, as was 1600. But 1700, 1800 and 1900 are not leap years. "It seems a bit arbitrary," says Ian Stewart, emeritus professor of mathematics at Warwick University. But there's a good reason behind it.
"The year is 365 days and a quarter long - but not exactly. If it was exactly, then you could say it was every four years. But it is very slightly less." The answer arrived at by Pope Gregory XIII and his astronomers when they introduced the Gregorian calendar in 1582, was to lose three leap days every 400 years. The maths has hung together ever since. It will need to be rethought in about 10,000 years' time, Stewart warns. But by then mankind might have come up with a new system.
4. Why isFebruary 29, not February 31, a leap year day? All the other months have 30 or 31 days, but February suffered from the ego of Roman Emperor Caesar Augustus, says Stewart. Under Julius Caesar, February had 30 days, but when Caesar Augustus was emperor he was peeved that his month - August - had only 29 days, whereas the month named after his predecessor Julius - July - had 31. "He pinched a couple of days for August to make it the same as July. And it was poor old February that lost out," says Prof Stewart.
5. The traditionof a woman proposing on a leap year has been attributed to various historical figures. One, although much disputed, was St Bridget in the 5th Century. She is said to have complained to St Patrick that women had to wait too long for their suitors to propose. St Patrick then supposedly gave women a single day in a leap year to pop the question - the last day of the shortest month. Another popular story is that Queen Margaret of Scotland brought in a law setting fines for men who turned down marriage proposals put by women on a leap year. Sceptics have pointed out that Margaret was five years old at the time and living far away in Norway. The tradition is not thought to have become commonplace until the 19th Century.
It is believed that the tradition of women proposing on this day goes back to the times when the leap year day was not recognised by English law. Under this theory, if the day had no legal status, it was acceptable to break with the convention of a man proposing.
6. A prayerhas been written by a female cleric for people planning a leap year day marriage proposal. The prayer, for 29 February, asks for blessings on the engaged couple. It reminds them that wedding plans should not overtake preparations for a lifetime together. The prayer has been taken from Pocket Prayers of Blessing by the Venerable Jan McFarlane, Archdeacon of Norwich:
"God of love, please bless N and N as they prepare for the commitment of marriage. May the plans for the wedding not overtake the more important preparation for their lifetime together. Please bless their family and friends as they prepare for this special day and may your blessing be upon them now and always. Amen."
7. The practiceof women proposing in a leap year is different around the world. In Denmark, it is not supposed to be 29 but 24 February, which hails back to the time of Julius Caesar. A refusal to marry by Danish men means they must give the woman 12 pairs of gloves. In Finland, it is not gloves but fabric for a skirt and in Greece, marriage in a leap year is considered unlucky, leading many couples to avoid it.
8. The chanceof being born on a leap day is often said to be one in 1,461. Four years is 1,460 days and adding one for the leap year you have 1,461. So, odds of 1/1,461.
But Stewart points out that is very slightly out, owing to the loss of the three leap years every 400 years. In any case, babies are more likely to be born at certain times of the year rather than others, due to a range of other factors, he says. Babies born on 29 February are known as "leapers" or "leaplings".
9.Other calendarsapart from the Gregorian require leap years. The modern Iranian calendar is a solar calendar with eight leap days inserted into a 33-year cycle. The Indian National Calendar and the Revised Bangla Calendar of Bangladesh arrange their leap years so that the leap day is always close to 29 February in the Gregorian calendar.
10.Explorer ChristopherColumbus used the lunar eclipse of 29 February 1504 to his advantage during his final trip to the West Indies. After several months of being stranded with his crew on the island of Jamaica, relations with the indigenous population broke down and they refused to continue helping with food and provisions. Columbus, knowing a lunar eclipse was due, consulted his almanac and then gathered the native chiefs on 29 February. He told that God was to punish them by painting the Moon red. During the eclipse, he said that God would withdraw the punishment if they starting co-operating again. The panicked chiefs agreed and the Moon began emerging from its shadow.
Also of a supernatural nature, on 29 February 1692 the first warrants were issued in the Salem witchcraft trials in Massachusetts.