Take A Leap

Tomorrow is February 29th, or as some call it Leap Day, a day in which you can do anything and it won't count negatively against you. It theoretically occurs every four years, making those years in which it occurs understandably called a Leap Year. It's all done in order to keep our calendars in sync with Earth's physical orbit around the Sun.

While our calendar in a normal year consists of 365 days, a complete revolution around the Sun in fact takes closer to 365 days and 6 hours. Every four years, when the total of missed hours equals 24, we simply add an extra day to the calendar to make up for it.

Although, even that number is inaccurate, since it is rounded up for ease of use. One revolution around the Sun is more precisely 365 days, 5 hours, 49 minutes, and 16 seconds. To compensate for this fact, an end-of-the-century year is not considered to be a leap year unless it is also exactly divisible by 400.

The concept of having a leap day was first introduced in 45 BC when Julius Caesar reformed the Roman Calendar and added a total of 10 extra days to the year. The reform was brought about in order to approximate themselves with the solar year, which is based on the Sun and the seasons experienced on Earth.

Therefore, two extra days were added to January, August, and December, while one extra day was added to April, June, September and November. February remained unchanged and stayed at 28 days, which was a traditional month length at the time.

But the Julian calendar is not what we use today; it was reformed again in 1582 by Pope Gregory XIII and that is why what we actually use today is called the Gregorian calendar. The Gregorian reform was enacted because the Julian calendar's arrangement of counting one year as 365 days and 6 hours, with a leap day every 4 years, allowed for us to gain 3 days every 400 years and throw us off our astronomical alignment, effectively screwing up the timing of the seasons.

For the Gregorian calendar, this error was figured out and a year was shortened by 11 minutes to the more accurate 365 days, 5 hours, and 49 minutes. Because of that change, it was proposed that the number of leap years in four centuries be reduced from 100 to 97, instead of occurring every 4 years no matter what.

Thus is why now, with our Gregorian calendars, we follow this rule: "Every year that is exactly divisible by four is a leap year, except for years that are exactly divisible by 100; the centurial years that are exactly divisible by 400 are still leap years." For example, the year 1900 is not a leap year; the year 2000 is. 

So in the long run then, a leap year won't actually occur every four years. Since most of us don't live long enough, or aren't lucky enough to exist during a centurial year not divisible by 400, most will never experience the rule's exception in skipping a leap year.

It's just easier to say that a leap year simply happens every four years, instead of going into a lengthy explanation like I just have. Regardless, no calendar will ever be perfectly accurate, as there are cosmic forces always effecting the amount of time it takes Earth to complete a revolution around the Sun.