Leap Year

Because the Earth takes a little more than 24 hours to spin around the sun, we end up with a little less than a quarter day every year extra. This spare change has been a problem, one that we have to continually keep under control. I wanted to share the calculation with you because I read it recently and was surprised at how complex it was.

From Wikipedia:

A leap year is a year containing one or more extra days in order to keep the calendar year synchronized with the astronomical or seasonal year. For example, February would have 29 days in a leap year instead of the usual 28. Seasons and astronomical events do not repeat at an exact number of full days, so a calendar which had the same number of days in each year would over time drift with respect to the event it was supposed to track. By occasionally inserting an additional day into the year, the drift can be corrected. A year which is not a leap year is called a common year. In fact, the Earth takes slightly under 365 1/4 days to revolve around the Sun.

The Gregorian calendar, the current standard calendar in most of the world, adds a 29th day to February in all years evenly divisible by 4, except for centennial years (those ending in -00) which are not evenly divisible by 400. Thus 1600, 2000 and 2400 are leap years but 1700, 1800, 1900, 2100, 2200 and 2300 are not.

The reasoning behind this rule is as follows:

• The Gregorian calendar is designed to keep the vernal equinox on or close to March 21, so that the date of Easter (celebrated on the Sunday after the 14th day of the Moon that falls on or after 21 March) remains correct with respect to the vernal equinox.^[1]
• The vernal equinox year is currently about 365.242375 days long.
• The Gregorian leap year rule gives an average year length of 365.2425 days.

This difference of a little over 0.0001 days means that in around 8,000 years, the calendar will be about one day behind where it should be. But in 8,000 years, the length of the vernal equinox year will have changed by an amount which can not be accurately predicted. Therefore, the current Gregorian calendar suffices for practical purposes.

There is a tradition, said to go back to Saint Patrick and Brigid of Kildare in 5th century Ireland, but apparently not attested before the 19th century, whereby women may make marriage proposals only in leap years.

Supposedly (but disputed), in a 1288 law by Queen Margaret of Scotland (then age five and living in Norway), fines were levied if the proposal was refused by the man; compensation ranged from a kiss to £1 to a silk gown, in order to soften the blow.^[7] Because men felt that put them at too great a risk, the tradition was in some places tightened to restricting female proposals to 29 February.

Others regard these supposed folk traditions as unhistorical.^[8]

In Greece, it is believed that getting married in a leap year is bad luck for the couple. Thus, mainly in the middle of the past century, couples avoided setting a marriage date in a leap year.