Time

Why do we have leap years?
Answered by Science Channel
  • Science Channel

    Science Channel

  1. The typical Earth year features 365 days. That's the number of days (or rotations around Earth's axis) it takes our planet to complete one orbit of the sun. In reality, though, 365 days is a rounded-off number. That's because the Earth actually takes 365.242199 days to orbit the sun. Throw in an extra day every four years, and you get a little closer to the truth: 365.25 days per year. Still, you don't have to be a math genius to see that an extra day every four years is an overcorrection. To even things out, we tinker with the regularly scheduled leap year that occurs at the dawn of each new century. Cancelling the leap day during years such as 1700, 1800 and 1900, but then having one every fourth century (such as the year 2000) as planned, sets the calendar back on track.

    We owe this idea to the ancient Romans, who first devised a way to make the calendar manageable but closely match astronomy and the natural seasons and moon phases. They set years at 12 months that stretched over 355 days. Eventually the calendar would be off from the moon and sun, though, so the resourceful Romans would add an extra month of 22 days (for 377 total) when the mood hit them. When Julius Caesar came into power about 46 B.C., he introduced a new calendar based on 365 days and a spare every four years -- the basis for the current Julian calendar and leap year [sources: Forbes, Connecticut State Library]. The Gregorian calendar modified Caesar's formula slightly in the Middle Ages to compensate further.

    Even though the leap year formula isn't perfect, it seems to work; a leap year certainly beats trying to plan meetings, school days and doctor appointments around a day that ends with an odd number of minutes, or trying to insert leap seconds or minutes throughout every calendar year. The only real drawback to leap year falls on the rare souls born on February 29 -- the so-called "leaplings" -- who have to decide whether to celebrate a day early or a day late every four years. Plenty of others find leap years as causes to celebrate. For example, Disney's parks in California and Florida planned to stay open 24 hours on February 29, 2012, to celebrate the leap year [source: Munarriz].

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