Leap Years: Why?

Leap Years: Why?

This year brings another February 29. Why do leap years exist? Jim Sowell is a senior academic professional in the School of Physics and the director of the Georgia Tech Observatory. He says leap year was a creation of Julius Caesar.

This year brings another February 29. Why do leap years exist? Jim Sowell is a senior academic professional in the School of Physics and the director of the Georgia Tech Observatory. He says leap year was a creation of Julius Caesar.

As of February 29, 2016, the calendar is back on track. Every four years, the addition of one day brings us closer to a more precise calculation of the Earth’s orbit.

Getting to our current calendar system has been a complicated process, mostly because of three important celestial rotations: the Earth’s spin, which gives us the day; the orbit of the Earth around the sun, which gives us the year; and the orbit of the moon around the Earth, the basis for the month.

The Romans developed one of the first calendars, which was primarily based on the moon’s orbit and had 10 months. March was first. December was last. We know this because the root of “December” means “ten” in Latin. By 70 B.C., two more months had been added, and February was last.

During the reign of Julius Caesar (46 – 44 B.C.), the Egyptians were some of the best astronomers. They could see that the Earth’s orbit was very close to 365.25 days. Caesar used their knowledge to set order to the calendar. He established that every fourth year would be a leap year, tacking one day onto the last month, February. In 45 B.C., he moved January to the beginning, thus establishing the Julian calendar. July was later named for him after his death. 

In the late 1500s, astronomers under Pope Gregory XIII (for whom the modern Gregorian calendar is named) realized 365.25 days wasn’t exactly right. The Earth’s orbit is actually 365.242199 days. After hundreds of years using the Julian Calendar, spring had begun moving into winter. The astronomers worked out a new formula to skip some leap days to get closer to an exact measurement. They also took 10 days out of the calendar in 1582 to get back on track. 

The Gregorian calendar will work for a long time — at least for the next 3,300 years. Then a new calculation will be needed to get our calendar in sync with the Earth’s orbit. Today’s astronomers aren’t sure what that will be, but we’ve decided to let future astronomers figure it out.

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Jason Maderer
National Media Relations
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jason.maderer@comm.gatech.edu