The upcoming February 29 leap day is added to our calendar once every four years and the math is pretty straightforward: Since the earth takes approximately 365.25 days to orbit the sun, and since a day can only be a day–not a quarter of a day–a year is normally 365 days. Then, every fourth year we add a day to the end of the month of February so the year will have 366 days. Voila! 

This way of keeping our calendar in line with the seasons was first suggested by the Egyptians about 300 years before Christ, but it wasn’t until Julius Caesar came along that the idea was put into practice. Therefore, on January 1, 45 BC (or, as the Romans would have thought of it, January 1, 709 years after the founding of Rome), Rome made the much simpler solar calendar the law of the land. 

But why did Caesar decide to do that, and what is the problem it solved?

The Lunar Calendar

Many religious festivals follow the cycles of the moon. There are new moon and full moon festivals, and the Jews announced the beginning of every month with the blast of trumpets.

Such events are a major feature of every civilization since time began, and such civilizations as the Chinese, the Egyptians, the Babylonians, the Olmecs, the Greeks, the Romans, the Celts, etc. all cared about the first appearance of the new moon and the beauty of the full moon. They would set their calendars by it, hence months are about 30 days long (although 29.5 days is mathematically more accurate). 

However, such devotion presented a major dilemma because a lunar year of 12 months is only about 354 – 355 days long, and a solar year is 365.25 days long. That’s 10 – 11 days off every year, so, if one wants to have the months to continue to match up with the seasons from year-to-year, then one must make adjustments. But why did adjusting the calendar matter to them so much?

Holy Days

It was because they took seriously the holy days they set aside to honor the gods. It was a matter of great importance to them. Gavin White in his book, Babylonian Star-Lore, points out that,

The earliest sources of information on the calendar are economic records that detail the distribution of goods to and from the temple….

Archetypal time manifests itself as much in the birth, life and death of a day as it does in the revolution of the year. It not only describes the life-cycle of every sentient being but also the grand cycle of creation itself, from the birth of the world-order from the waters of creation, all the way to its ultimate destruction .

The order and pattern of nature–and even of history–as seen in the movement the stars was so important that we developed calendars so that we, too, could bring order and continuity to our civilizations. Therefore, the lunar and solar calendars were to be kept in sync for religious reasons and to make sure that the holy day celebrations were set apart to honor the gods in their proper seasons, or, in the case of the Jews, the one true God (Leviticus 23). 

Caesar’s Adaptation

However, as we have seen, keeping the lunar and solar calendars in line was difficult, so, with the help of an Egyptian named Sosigenes, Caesar decided to quit making it so complicated and to just focus on the solar calendar. The advantage was that the priests could still do their thing and pay attention to the cycles of the moon for their religious celebrations, but now the government could do their thing and work their bureaucracy much more efficiently.  Adding a day every four years was super simple compared to the old way, and it would override all the complexities that were caused when having to coordinate important events with other nations who had calendars of their own. The nations Rome conquered could still use their own lunisolar calendars and worship their own gods, but now they also had to use the much more efficient solar calendar of the Romans. It was a prudent and practical solution to a managerial nightmare, and we use his solution to this day–almost.

It turns out that Caesar’s changes were a bit too simple.

The Gregorian Calendar

Fifteen hundred years after Caesar’s changes, Pope Gregory XIII was tasked by the Council of Trent to modify the Julian Calendar because it had drifted out of alignment with the seasons by ten days. It turns out that because a year is actually 365.2422 days, or to be over-the-top precise, 365.2421897 solar days, even the simple adjustment of a leap day every four years wasn’t adequate.

To illustrate, the spring equinox–which used to occur on March 21 when Caesar made his changes–had shifted to March 11 by the 1500’s. That would cause managerial problems for the government and it made calculating the date of the resurrection more difficult for the church. Therefore the Church decided to slightly tweak the way the Julian Calendar did leap years.

What was the change Pope Gregory XIII implemented in February of 1582? After two decades of work, the solution was simple and elegant: They would eliminate a leap year every hundred years, or so:

“The rule is that if the year is divisible by 100 and not divisible by 400, the 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” (Air and Space).

They also had to subtract ten days from the calendar–just gone–but these modification brought everything back into alignment and made sure that our calendar wouldn’t drift anymore–at least not for a long time. It will need to be corrected again at some future date, but we’ll let future generations deal with that.

But Why February?

By the way, as a matter of interest, have you ever been curious as to why we add the leap day at the end of February? Why February? Why not July or August? What is special about February?

Well, February was chosen because before the calendar changes made by Numa Pompilius (the successor to Romulus around 700 BC), February had been the last month of the traditional Roman lunar calendar. March used to be the first month and the Romans would add an intercalary month every two or three years, called Mercedinus in between February and March.

In other words, the Romans put their calendrical adjustments at the end of their year, which at one time was February.  Numa chose to make January the first month  (see my post: Why is New Years on January 1?), but he continued the practice of putting calendrical adjustments at the end of February even though it was now the second month instead of the last month. Whew!

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