Since today, a leap day occurs only once every four years, I thought I might like to write a little about why we have leap years and where the idea originated. Our calendar ultimately comes from the calendar used by the Romans. The names of the months and the number of days in each month are basically the same, though the year originally began in March and the Romans did not count the days from the beginning of the month but counted backward from three fixed days, the kalends, the nones, and the ides.
The Roman calendar was, like many ancient calendars, a lunisolar calendar with an intercalary month added at intervals to keep the dates aligned with the seasons. The responsibility for inserting the intercalary month lay with the Pontifex Maximus, the leader of the order of Priests called the Pontiffs. (One of the titles of the Pope is the Pontiff.) Unfortunately, this position was a political one and the Pontiffs got in the habit of inserting the extra month to prolong the terms of their political allies, or not inserting it if their enemies were in office. By the time of Julius Caesar, the date was three months behind the seasons.
In 46 BC, Caesar returned to Rome from Egypt. The Egyptians had long used a solar calendar of 365 days. Caesar brought mathematicians and astronomers from Alexandria with him and he directed them to reform the Roman calendar. The calendar they developed is called the Julian Calendar. In this new calendar, they changed the first month to January and gave each month the number of days it now contains. Most importantly, they did away with the intercalary months altogether. The Julian calendar was to be solely a solar calendar and the months would have no relation to the moon. Caesar lengthened the year 46 BC to 445 days to bring the date back in alignment with the seasons. This year was called the year of confusion, but it was the last year of confusion as the Julian calendar was adopted throughout the Roman world and is used with some modifications to this day.
The most important reform the Greek astronomers made was the introduction of the Leap Year. The problem is that the year is not exactly 365 days. Instead, as the astronomers had learned, the year is closed to 365 1/4 days. So, it seemed that an easy way to keep the date aligned was to simply add a day every four years. And so, since Caesar’s reform of the calendar, we have had leap years every four years.