Why do we have leap year?


David Kieda

In Denmark, the tradition is that women may propose on the bissextile leap day, February 24, and that refusal must be compensated with 12 pairs of gloves. 

In Finland, the tradition is that if a man refuses a woman’s proposal on leap day, he should buy her the fabrics for a skirt. 

In Greece, marriage in a leap year is considered unlucky. One in five engaged couples in Greece will plan to avoid getting married in a leap year.

From Wikipedia

FYI News
asked David Kieda, professor of physics and chair of the Department of Physics and Astronomy, to give us some insight on why a leap year has been integrated into our calendar.

FYI News: Why do we have leap year?
David Kieda: The leap year is in place so that common astronomical events occur on the same calendar day every year (or close to it). The important events are the summer and winter solstices and the spring and fall equinoxes. This would be easy if the time for the Earth to complete one orbit was an exact number of days. Unfortunately, it is not; one exact orbit around the sun takes 365 days, five hours, 49 minutes and 12 seconds which is 365.2425 days (incidentally, this can happen; Mercury’s orbital period and rotational period is locked in a 2:3 resonance, meaning it has exactly three days for exactly two orbits around the sun!)

FYI: What effect does that extra time have on Earth?
So the consequence for Earth is that after four orbits (years), 1490.97 days have passed. We account for that extra 0.96 by adding one leap day every four years in order to keep the solstices and equinoxes occurring on the same calendar date each year.

FYI: What would happen if we didn’t add that extra day?
Kieda: If you did not do this, then after 400 years the summer solstice would be occurring about 100 days earlier in the calendar: the calendar would say spring, when in fact it was actually summer outside.

By the way, notice the calendar still isn’t perfect…every four years you are 0.03 days short. After a century (25 leap years) you will be short by 0.75 days. So to make up for this, on the year of the century (years 1700, 1800, 1900, 2100, 2200) there is no leap day (I’ll bet you did not know that!).

Every century you are now going forward by 0.25 days. So every 400 years, you need to have an extra day. So there is a century rule: The century years do not have a leap day, unless they are divisible by 400, in which case there is a leap day.

Which explains why there was a leap day in the year 2000…the first one in 400 years!


4 Responses to Why do we have leap year?

  • Roland Loewer says:

    That was interesting.The photo showed you against a blackboard with lots of math formulas. Oh,no…here comes the complicated stuff. Thanks for explaining it in an understandable way. It makes perfect sense now. Thanks.

  • cornelio says:

    so did you guy skipped the year 2000? In you article you said that the year 1700,1800,1900, 2100 and 2200;

  • floor says:

    David Kieda says this: Please let the reader know
    that yes we skipped 2000…that was intentional.
    That falls under under the 400 year rule (last line of the article). All the centuries provided in that list are NOT divisible by 400……1600, 2000, 2400 are, and so they are NOT on that century list….

  • Pris says:

    Hi David, maybe you can help me, I’ve been seeing this post around the internet and I’m pretty sure it’s wrong, it states: “There have been about 514 leap years since Caesar created it in 45BC. Without the extra day every 4 years, today would be July 28, 2013. Also, the mayan calendar did not account for leap years, so technically the world should have ended 7 months ago.”

    Okay I for one don’t believe in the 2012 end of the world stuff, but they did calculate the amount of leap years wrong right? Based on the math I did, there have only been 511.25 leap years, because we skipped 3 since 1583 when the Gregorian calendar came out, right?

    I did simple math and correct me if I’m wrong, add 45 (which is 45 BCE) to 2012, that gives you 2057, (which is 2057 years since Cesar started the leap year rule) divide that by 4 and you get 514.25, subtract the 3 years skipped since then(1700, 1800, and 1900) and you get 511.25, so there, there have actually only been 511.25 leap years not 514.

    Again, correct me if I’m wrong. Thank you.

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