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Leap Year: Why Do We Jump An Extra Day Every Four Years?

Why do we have leap years?

Every four years, an extra day is added at the end of February, which helps us remedy the disparity of the Gregorian calendar. To give a little background to the disparity, a complete orbit of the earth around the sun takes exactly 365.24219 days to complete, but the Gregorian calendar uses 365 days which means our calendar is out by around a quarter of a day a year.

To get everything back on track, leap seconds and leap years are added every four years as means of keeping our clocks (and calendars) in sync with the Earth and its seasons.

Julius Caesar vs Pope Gregory

The Roman calendar used to have 355 days with an extra 22-day month every two years. When Julius Caesar became emperor in the 1st Century, he ordered his Alexandrian astronomer Sosigenes to devise something better. To make the adjustment, Sosigenes decided on a 365-day year with an extra day every four years to incorporate the extra hours, and thus February 29th was born.

However, the system was tinkered with again by Julius Caeser about 500 years later. In 45 B.C.E. the Julian calendar was created and included the practice of adding an extra day every four years, making up for those quarter days.

A solar year isn’t exactly 365 ¼ days – 365.242 – to be exact, and over time that adds up which would through the days to the seasons again. By 1582 A.D. that slight discrepancy in the Julian calendar added up to 10 days. Pope Gregory XIII recognized this and created the Gregorian calendar, 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.

Enjoy your extra day and make the most of it!