It happens just about every fourth year. Leap Year -- that magical time when we schedule an extra day in the year to make up for our failure to calculate time correctly, is here once again, and when February 29 comes and goes this year, we will have helped to make up for lost time. Make sense? No, probably not. It’s confusing at best, even when we understand it. Let me explain. The Gregorian calendar, which now serves as the standard calendar for civil use throughout the world, has both common years and leap years. A common year has 365 days and a leap year 366 days, with the extra, or intercalary, day designated as February 29. A leap year occurs every four years to help synchronize the calendar year with the solar year, or the length of time it takes the earth to complete its orbit about the sun, which is about 365¼ days. The length of the solar year, however, is slightly less than 365¼ days—by about 11 minutes. To compensate for this discrepancy, the leap year is omitted three times every four hundred years. In other words, a century year cannot be a leap year unless it is divisible by 400. Thus 1700, 1800, and 1900 were not leap years, but 1600, 2000, and 2400 are leap years. The Gregorian calendar is closely based on the Julian calendar, which was introduced by Julius Caesar in 45 BC. The Julian calendar featured a 12 month, 365 day year, with an intercalary day inserted every fourth year at the end of February to make an average year of 365.25 days. But because the length of the solar year is actually 365.242216 days, the Julian year was too long by .0078 days (11 minutes 14 seconds). This may not seem like a lot, but over the course of centuries it added up, until in the 16th century, the vernal equinox was falling around March 11 instead of March 21. In 1582, Pope Gregory XIII adjusted the calendar by moving the date ahead by 11 days and by instituting the exception to the rule for leap years. This new rule, whereby a century year is a leap year only if divisible by 400, is the sole feature that distinguishes the Gregorian calendar from the Julian calendar. It’s obvious that our problem with keeping up with time is not new. We are told that mathematics are faultless, which would mean the problem, then, must lie with our inability to do math very well. But in spite of our shortcomings, we have learned to “leap ahead” - which is exactly what we are doing this year. Understand it or not -- now we're “fixed”, so breathe easy for another four years and we'll do it all over again.