Leap Year 2012: Why We Need February 29
Leap day added to correct "sloppy" calendar drift.
But where did leap year come from? How does it work? And have other cultures with their own systems for tracking time needed to use it, too?
We observe the modern leap year because Earth orbits the sun every 365.242 days—not an easy number for a calendar to accommodate. (See an interactive map of the solar system.)
As a result, many cultures since ancient times have taken on the practice of adding extra days, or even months, to round out the calendar year.
(Related: "New Calendar Would Add Extra Week to December.")
Early calendars were often based on lunar months, which average 29.5 days. But a year of such months totals only about 354 days.
This discrepancy resulted in annual events—festivals, agricultural milestones,
