Why do we have leap years? And why does it only happen every four years?
The short answer is... SCIENCE!
A solar year is the time it takes for the Earth to make one complete revolution around the sun. A solar year takes approximately 365 days, 5 hours, 48 minutes, and 46 seconds.
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The years that we have 365 calendar days, we still have the extra 5 hours, 48 minutes, and 46 seconds. Just because we skipped it doesn't mean that it goes away, in fact it accumulates (a fancy word for builds up over time). After four years that adds up to a full day.
What would happen if we didn't observe leap day every four years?
The whole calendar would be messed up, seasons would no longer match up to the correct months.
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So to recap, we add an extra day to the calendar every four years. February 29th is known as leap day and the year in which it occurs is known as a leap year.
When did leap year officially start?
In 45 B.C.E. Julius Caesar, the Roman dictator, reformed the imprecise Roman calendar to the 12 month, 365-day format. This included the additional leap day every four years.
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However...
There was still one major issue. The Julian calendar year still did not match up with the solar year. The Julian year was 364 days, 6 hours and remember the Earth orbiting the sun with the solar year being 365 days, 5 hours, 48 minutes, and 46 seconds.
This means that the calendar was still gaining a full day every 128 years. This might not seem like a big deal, but after more than 1,000 years it really started to add up. So much so that by the late 1500s the vernal equinox, or the first day of Spring, which was suppose to fall on March 21st was moving closer to February.
All of the lost time was making it difficult to set the dates of religious festivals. In 1582, Pope Gregory XIII announced a change. In the Gregorian calendar, there would still be a leap day every four years, with one important exception. Years ending in "00" would not be leap years anymore, except when divisible by 400.
So the long answer to leap year is... MATH!
Be prepared because in the year 2100 we will not observe leap day.
There you have it! The calendar system that we use today! It is not perfect, the calendar still loses a day every 3,200 years or so.
FUN EVENTS ON LEAP DAY!
Celebrate your extra day with extra fun in Ann Arbor!
PAPER DRAGONFLY, BUMBLE BEE, AND BUTTERFLY PUPPETS - WG
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