With 2012 being a leap year, it is a good time to remind ourselves why it is that we add an extra day to our calendar almost every four years.

The answer, simply put, is that the Earth doesn't take exactly 365 days to orbit the sun. It's actually closer to 365.242 days. While that may not seem like much, it amounts to almost six extra hours a year, which does add up over the long run.

A leap year helps to prevent the shift that would happen in our calendars because of these extra hours. Without it, our calendar would eventually fall out of sync with the weather. In 100 years, our calendar would be about 24 days off, and before we knew it, we'd be seeing winter-like conditions in July and have warm, sunny Decembers.

Ancient civilizations had a variety of ways to accommodate for this — after all, harvests, rituals, and celebrations all were pegged to specific days on a calendar. Many of the accommodations involved complicated maneuvers that often made things even worse, like adding a month to certain years.

The idea of a leap day was introduced in the first century BC by Roman leader Julius Caesar, who decided that every fourth year would have an extra day added to offset the shift. While this was a good plan, it wasn't perfect. Given that each year had only an extra .242 of a day, over time, the extra day added every four years also threw the calendar off — by one day every 128 years.

In 1582, the Gregorian calendar introduced by Pope Gregory XIII remedied this problem by adopting new rules for calculating leap years, which we use to this day:

• To be a leap year, the year must be evenly divisible by four.
• Any year that is evenly divisible by 100 is not a leap year — unless it is also evenly divisible by 400.

### Other calendar systems

While most Western cultures follow the Gregorian calendar, other calendars approach leap years differently:

• The Jewish calendar adds leap months to seven years within a 19-year cycle. In Hebrew, these years are referred to as Shanah Me'uberet — translated in English as a "pregnant year."
• The Chinese calendar also makes use of leap months. Based on a lunar cycle, a leap year is added if there are 13 full moons between the 11th month of the first year and the 11th month of the following year
• The Ethiopian calendar is based on the ancient Egyptian calendar and adds a leap year every four years. It is about seven to eight years behind the Gregorian calendar. Ethiopians celebrated the new millennium on Sept. 12, 2007.