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Leap Year Rules, Facts, and Trivia

February 29th, 2008

leap-year-list.jpgAs you can see from the date, this is February 29, a “leap day” that will not occur for another four years. Keeping this in mind, I figured that I might as well do an educational post on this topic, especially after a quick Wordtracker scan revealed that a surprising number of you are searching for leap year-related information. Therefore, I will address some of the most common questions and provide you with specific answers that I have obtained from my research.

Why do we have leap years?

The reason for leap years is to keep our current calendar in alignment with the astronomical solar years, which occur every time the Earth makes a full revolution around the sun. Specifically, the actual solar year is 365 days, 5 hours, 48 minutes, and 46 seconds long, or 365.2423 days in decimal terms. It is calculated by taking the average time that it takes for the Earth to move from one vernal (spring) equinox to the next, thus completing one full orbit of the sun.

What is the history of leap year? How did we come up with the current system?

By 46 BC, the Romans, who had devised an ancient calendar that roughly kept pace with the changing seasons, figured out that they could not simply make each year 365 days long because the extra fraction of a day would cause the calendar to be quite inaccurate after a few decades. Borrowing the original idea from the Egyptians, they were able to compensate for most of this difference by adopting the Julian calendar (named after Julius Caesar), which inserted an extra “leap day” in February every four years. This became the standard calendar for most of Western civilization for the next 1500+ years.

However, this practice of simply adding in the leap day every four years, effectively making the calendar year 365.25 days long, still allowed for a small rounding error to creep in, thus causing a noticeable inaccuracy after several centuries had passed. This led to the adoption of the Gregorian calendar in 1582 (some countries did not make the switch until 1712 or 1752), which introduced a slight twist to the usual leap year rules. For every 100 years, leap days were removed, except for years that were divisible by 400. This meant that 1600 was still a leap year, but 1700, 1800, and 1900 were not.

This latest refinement to the chronological system greatly reduced (but did not completely eliminate) the difference between the astronomical and calendar year lengths, making the current calendar sufficiently accurate for modern practical purposes. Theoretically, the calendar may need to be adjusted again after another 8,000 years or so, but by this time other astronomical factors such as tidal acceleration and precession of the equinoxes come into play, making it almost impossible to predict at this point whether or exactly how much of an adjustment may be needed.

What is the leap year calculation? How do I calculate the rules for leap year?

The rule for determining leap years is simply this: every year that is divisible by four is a leap year, except those that are divisible by 100 but not 400. For practical purposes going forward, this means that leap years will occur every four years from now (2012, 2016, 2020, etc.) until we reach 2100, which will NOT be a leap year. However, because of the limitations of human life spans, it is unlikely that anyone reading this today will live long enough for this to be a concern. But if you do manage to survive until the end of the 21st century, just remember that February 2100 will only have 28 days.

Was 2000 a leap year?

Yes, because it is divisible by 400.

Was 2004 a leap year?

Yes - it is divisible by four and does not qualify for the exception.

Is 2006 a leap year?

No, because it is not divisible by four.

Is 2008 a leap year?

Yes, for the same reasons as 2004. In fact, today is the leap day (February 29), which is why I’m writing this article!

When is the next leap year?

After today, it’s 2012. It will continue to occur every four years thereafter until we reach 2100, by which time you’ll probably be too old to care about it anyway.

What are some other bits of leap year trivia?

  1. The first calendar that made a provision for leap year occurring every four years was introduced by King Ptolemy III of Egypt in 238 BC.
  2. In 1582 AD, the countries of Italy, Portugal, Spain, and Poland were the first to switch over from the Julian to Gregorian calendars, and did so by dropping 10 days from October of that year.
  3. Great Britain (including the American colonies at that time) did not switch to the Gregorian calendar until 1752, by which time they had to drop 11 days out of October to bring the calendar into sync with the astronomical year.
  4. Sweden, intending to switch to the Gregorian calendar in 1700, did not have a leap day for this year, which was correct according to the new rules. However, they somehow forgot to observe the leap days in 1704 and 1708, which put them out of sync with both the Julian and Gregorian calendars. In order to correct this mistake, they had a “double leap day” in 1712, thus creating the unique day of February 30 and effectively putting them back on the Julian calendar.

    The Swedes eventually completed the transition to the Gregorian calendar in 1753 by dropping 10 days out of February, going directly from February 17 to March 1. Interestingly, some of them actually opposed this reform of the calendar because they believed that they were losing 10 days from their life spans and would therefore die sooner. It is not known whether the relatively high presence of blondes found in Sweden had anything to do with this unusual sequence of events.

  5. The Iranian (Persian) calendar, which is used today by Iran and Afghanistan, generally follows the practice of inserting a leap day every four years as the Julian calendar does, except that every 33 years (approximately), they make the necessary correction by allowing a five-year interval between leap years. Interestingly, this calendar system is actually more accurate than the Gregorian one because it is based on the actual observations of the vernal equinox from Tehran, and adjustments in the leap year time intervals are done accordingly.
  6. One oft-cited tradition regarding leap year is that women in the Middle Ages were allowed to propose marriage on February 29 (normally tradition dictated that women had to wait for the man to propose), every four years, leading to an occasional rush of females attempting to seduce males. Supposedly this tradition was started back in the 5th century by St. Patrick and St. Bridget in Ireland, but some historians dispute the validity of this, stating that there are no known mentions of this tradition until the 19th century.

    OH NOES!!11! I was born on February 29 - does this mean that I only get to celebrate my birthday once every four years?! This doesn’t seem fair…

    People who were actually born on the leap day can still celebrate their birthdays every year because it is not the date that you’re supposed to be celebrating, but simply the fact that you’re one year older. Therefore, you have a choice between having it on February 28 or March 1. Some people observe their birthdays on February 28 according to the reasoning that they were born on the last day of February regardless, so that is when they should celebrate. Personally, I would celebrate it on March 1 in common years (that is, non-leap years) because that is the date that would be February 29 if all years were leap years.

    Unfortunately, being born on a leap day does not grant you special powers such as a reduced rate of aging or a four-fold increase in your life span. Unless, of course, you are a character in a role playing game (RPG), in which case provisions for such powers may be implemented by your game’s programmer.


    Source Links: TimeandDate.com, InfoPlease.com, Inventors.About.com.


6 Responses to “Leap Year Rules, Facts, and Trivia”

  1. comment number 1 by: ireland5

    Very informative. I enjoyed reading your article - another in a long list designed to ’stamp out ignorance’.

  2. comment number 2 by: Wakish

    That’s a very good informtive article! Good effort! keep it up!

    PS: I thought about making an article on that occasion myself, but when I’m reading yours right now, I realize that I forgot completely about that..lol

    Best Wishes,
    - Wakish -

  3. comment number 3 by: kyh

    nice article! i always thought those who were born on feb 29th are so pity! LOL! now i’m crystal clear. :D

  4. comment number 4 by: Timothy

    I would like to know how often March 1st falls on a Sunday in a non-leap year. This makes for a perfect 4 week Februray calendar starting Sunday and ending Saturday.

  5. comment number 5 by: Karlonia

    @Timothy:

    This is an interesting question — I did some research on this earlier today and found out that next year (2009) happens to be one of those “perfect February” calendars you mentioned where the four weeks line up exactly from Sunday to Saturday with no partial weeks at the end. Later I found a list from TimeAndDate.com that shows the frequency of when this occurs:

    http://www.timeanddate.com/calendar/repeating.html?year=2009&country=1

    It seems that there is a general pattern of +6, +11, +11, +6, etc. that denotes the number of years before the exact calendar alignment repeats itself. There is probably some sort of mathematical formula for deriving the exact numbers, but I have not yet found one that is reliable for all possible years.

  6. comment number 6 by: Sandy

    I have a question if Feb 1 is on a Friday and the 29th is on Friday how long will it be before there is another February with 5 Fridays?

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