The ancient Egyptians noted the year was 365¼ days long. In the Roman calendar September, which comes from the Latin for seven, was the seventh month; October, after the Latin number eight, was the eighth month, etc. They then added January and February to the end of their calendar making 12 months. In 47 BC Julius Caesar developed the Julian Calendar where every fourth year February had 30 days. After his death the fifth month, Quintilius, was renamed Julius in his honor. Augustus Caesar later changed the sixth month from Sextilius to Augustus and took another day from poor February to make his month as long as Julius. Now, fast forward to 1563 when King Charles IX of France decided that the year should begin near the celebration of the birth of Christ. So he changed the beginning of the year from March to January. Now, September, named after the Latin number seven, became the ninth month, etc. With an extra day added for a leap year every fourth year, the average length of the calendar was 365.2500 days long. That’s an error of 0.0078 days per year. By 1582 this amounted to an error of ten whole days and Pope Gregory XIII set up by decree what we now know as the Gregorian Calendar. Here are his rules for leap year: • Years evenly divisible by 4 are leap years unless they are century years, i.e., years evenly divisible by 100. • Century years are not leap years unless they are evenly divisible by 400. • Years evenly divisible by 400 are leap years. Thus, 1900 was not a leap year but 2000 was. And 2012 is a leap year since it is evenly divisible by 4 and is not a century year. Oh, yes, a footnote to all this. In 1582 the Protestant countries did not go along with Pope Gregory’s decree nor had they changed the beginning of the year to January 1. It wasn’t until September 2, 1752 that those countries, including the American colonies, changed from the ancient Julian calendar to the Gregorian Calendar. About PARI PARI is a not-for-profit public foundation established in 1998. Located in the Pisgah National Forest southwest of Asheville, NC, PARI offers educational programs at all levels, from K-12 through post-graduate research. For more information about PARI and its programs, visit www.pari.edu. Follow PARI on Twitter at http://twitter.com/Astronomy_PARI. “Like” PARI on Facebook at www.facebook.com/Pisgah.Astronomical.Research.Institute.