This week’s leap year edition lands on Feb. 29. Leap years almost always occur every fourth year on the calendar. Having the extra day in February fall on a Thursday only happens once every 28 years.
According to the Smithsonian National Air and Space Museum: “A calendar year is typically 365 days long. These so-called ‘common years’ loosely define the number of days it takes the Earth to complete one orbit around the Sun. But 365 is actually a rounded number. It takes Earth 365.242190 days to orbit the Sun, or 365 days 5 hours 48 minutes and 56 seconds. This “sidereal” year is slightly longer than the calendar year, and that extra 5 hours 48 minutes and 56 seconds needs to be accounted for somehow. If we didn’t account for this extra time, the seasons would begin to drift. This would be annoying if not devasting, because over a period of about 700 years our summers, which we’ve come to expect in June in the northern hemisphere, would begin to occur in December! By adding an extra day every four years, our calendar years stay adjusted to the sidereal year, but that’s not quite right either.
Some simple math will show that over four years the difference between the calendar years and the sidereal year is not exactly 24 hours. Instead, it’s 23.262222 hours. Rounding strikes again! By adding a leap day every four years, we actually make the calendar longer by over 44 minutes. Over time, these extra 44+ minutes would also cause the seasons to drift in our calendar. For this reason, not every four years is a leap year. The rule is that if the year is divisible by 100 and not divisible by 400, leap year is skipped. The year 2000 was a leap year, for example, but the years 1700, 1800, and 1900 were not. The next time a leap year will be skipped is the year 2100.”
