The Origin of Leap Year and the Development of the Calendar

We have been keeping track of time for about as long as humans have been on the planet. At first, we just wanted to know what season it was at the moment and when the next one would come. But after the advent of agriculture, people realized that it was also important to keep track of every day of the year. Once you start trying to keep track of every day, you soon realize a problem: since the number of days in each year is not a whole number, we can’t make a calendar that remains the same and has the same number of days every year, hence today’s date!

We sometimes need to add an extra day to the calendar, so there is February 29th on the calendar. One major problem is that there is no relationship between the Earth’s revolution and its rotation. The time it takes the Earth to make one revolution along its axis of rotation is called a day, and the time it takes the Earth to make one revolution around the Sun is called a return year, and it would be nice if the Earth always returned to the same place in its orbit after a certain number of revolutions. In that case the number of revolutions would be the number of days in a year.

However, this is not the case, as the Earth rotates 365.2425 times during one revolution around the Sun, so if the calendar has 365 or 366 days per year, then you are either undercounting or overcounting. If you get it wrong then the longest and shortest days of the year usually occur at fixed points in the Earth’s rotation, namely the summer and winter solstices, and their positions on the calendar will change, making it difficult to prepare for events that take place on the same day every year. For example, the winter solstice celebrations.

That calendar was not perfect, so many ancient civilizations such as the Babylonians, the ancient Chinese and the Romans based their calendars on the moon, but the moon’s activity had nothing to do with the rotation or revolution of the earth, and the moon revolved around the earth in slightly more than 12 revolutions in the time it took for the earth to revolve around the sun, with each revolution taking a little over 29 days. Both calendars then faced the same problem: 12 lunar months a year was too few, but 13 lunar months a year was too many, so most people adopted a lunar calendar with 12 months a year, and in addition people would add a month to the lunar calendar every few years, so that important events such as harvesting crops would fall on almost the same day every year.

Then there were the ancient Romans who, very interestingly, did not have a proper calendar and left the decision as to how many years to add a month to it to the highest religious authority: the high priests. This tradition continued for some time, at least until the advent of Julius Caesar. The Romans considered the addition of extra months to be an ominous sign and they did not want to add extra months when they were at war, which Julius Caesar was doing all the time, so the Roman calendar became confused from then on. Eventually Julius Caesar decided that the Romans were to use a completely new calendar which should be systematic and specify which year the extra days should be added to, hence the Julian calendar. For a long time, this calendar was used in Western Europe and in the Christian churches, and most Julian years had 365 days, but as I said earlier the year had 365 and a quarter day, so every four years one day was added. This may seem reasonable, but it is not that simple.

As it turns out, this system of adding one day to every four years adds an extra ten minutes to the year, and the framers of the Julian year knew this. But they saw no need to complicate the calendar, it was only an error of 10 minutes a year. 10 minutes a year, but over the years the error was not insignificant. By the end of the 16th century, people had been using the Julian calendar for some 1600 years, by which time the calendar was already 13 days behind the actual date, until Pope Gregory XIII, the high priest of the Catholic Church at the time, decided to give the Julian calendar a major overhaul by combining the Grecian calendar, which we still use today, with the Julian calendar, which actually differs from the Grecian calendar in only a few ways.

The rules of the Gregorian calendar

The new rule in the Gregorian calendar was that every four years an extra day would be added, not if the year was divisible by 100, but if the year was divisible by 400. So 1700, 1800 and 1900 are not leap years because they are divisible by 100 but not by 400, and 2000 is a leap year because 2000 is divisible by 400. If we were still using the Julian calendar, 2016 would also be a leap year, and the Julian calendar would be 16 days behind the actual date. That’s no big deal.

Whether the calendar exists or not, the universe is still intricate, and the Gregorian calendar is still a bit too long, so we add a day to the calendar every 7700 years without expecting a new calendar to appear, and it was only when the Julian calendar was 13 days behind the actual date that people started using the Gregorian calendar. It will be about 100,000 years before the Gregorian calendar makes such an error, and we don’t care what it does in 100,000 years, there’s nothing we can do about it!