What goes around: The reason we have February 29 every four years is because of two different time cycles involved with the rotation of the Earth. Leap years happen every four years, except if the year is a multiple of But then years that are a multiple of are also leap years. This complex arrangement has a year history. The reason for an extra day in a leap year is that there are two fundamental time cycles both to do with rotation.
One is the time it takes for the Earth to rotate once on its axis — that's 24 hours, which is a day. The second is the time it takes the Earth to orbit the sun — which is By A. So Pope Gregory XIII created the Gregorian calendar, coined the term "leap year" and established February 29 as the official date to add to a leap year.
He also introduced a rule to take into account the discrepancy in the Julian calendar. Now, a leap year occurs in every year that is divisible by four, but only in century years that are evenly divided by Leap years are important so that our calendar year matches the solar year — the amount of time it takes for Earth to make a trip around the Sun.
But, if you keep subtracting almost 6 hours every year for many years, things can really get messed up. For example, say that July is a warm, summer month where you live.
If we never had leap years, all those missing hours would add up into days, weeks and even months. Eventually, in a few hundred years, July would actually take place in the cold winter months!
This is true of almost every other planet in our solar system. Mars, for example, has more leap years than regular years! A year on Mars is sols, or Martian days. February 29th is always the day added to the calendar on a leap year, and it's known as leap day. You can expect a leap year every year that's divisible by four, though with a few exceptions.
The Old Farmer's Almanac explained the math behind leap years. In essence, for a year to be a leap year, it must follow two rules. First, that it's evenly divisible by four and, second, years that are divisible by century years such as or can't be leap years unless they are also divisible by was not a leap year, but was. Make it easy on yourself, though, and just remember that, most of the time, it happens every four years or simply check a calendar. We need leap days to keep our calendars accurate.
As you remember from science class, a year is how long it takes for the Earth to circle around the sun. While in the Gregorian calendar, we've rounded that to days, it actually takes slightly longer than that—roughly While that may seem like a negligible difference, over time, those hours add up. To keep our calendar in check, we need to add in an extra day here and there to make up for that lost time—hence, the leap day.
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