On this day in 8 AD, the Leap Day concept as we know it officially went into effect.
In 46 BC, the Roman emperor Julius Caesar devised the Julian calendar to solve the discrepancy between the calendar year and the solar year, the time it takes the Earth to complete its orbit around the Sun. The Julian calendar did this by adding a day every four years, something the Egyptians had come up with to solve the problem.
The reason the extra day is necessary is because while there are 365 days in a calendar year, the solar year actually takes 365 days, 5 hours, 48 minutes and 46 seconds. It may not seem like much, but after four years, the calendar would fall behind the solar year by a day. After 100 years, the calendar would fall behind 25 days, meaning Summer would be starting a month late!
By the time the Julian calendar went into effect in 45 BC, the discrepancy had grown to the point that 10 extra days had to be added to that year to correct it. Then, the single extra day every four years began. Originally the day was added between the 23rd and 24th of the month of February, whereas there would actually be two days that counted as one.
Then in 8 AD, the Leap Day as an extra day added to the end of February occurring every year divisible by the number 4 began.
There was still a problem with this system, however. Even a century before the creation of the Julian calendar, Greek astronomers were aware that a year was slightly less than 365.25 days, as the Julian calendar in effect had it. To be precise, it’s 11 minutes and 14 seconds less than 365.25. So very gradually, much slower than before, the calendar drifted from the solar year again.
In 1582, the Gregorian calendar was introduced. It brought the calendar and solar years even closer into alignment by omitting 3 leap years every century. They did this on the years divisible by 100. So, 1700 was dropped as a leap year, as was 1800 and 1900. 2000 stayed a leap year, but 2100, 2200 and 2300 were no longer. 2400 stayed a leap year, and 2500, 2600 and 2700 did not. And so on.
With the Gregorian calendar, the difference between the calendar and solar years is 26 seconds. By the year 4909, the calendar year will be a full day ahead of the solar year.
In the 19th century, Sir John Herschel proposed a modification to the Gregorian calendar that would drop leap year every 4,000 years beginning in the year 4000. That modification, though often proposed, has yet to be accepted.