1582: The Year that Europeans Lost Ten Days

For over 1600 years, most European countries used the Julian calendar which was created by Julius Caesar in 46BC as a reform of the old Roman calendar. While the Julian calendar was not perfect, it proved to be fairly reliable. Over the years, however, its inaccuracies were magnified until it was as many as ten days off the solar year. In a time when agriculture was the backbone of all the economies in the world, it was vital to have a reliable way to reckon the time of year (and thus when crops should be planted). To that end, Pope Gregory XIII issued a papal bull in 1582 to reform the old Julian calendar.

One of the problems that one invariably encounters when trying to make a calendar is the inconvenient fact that solar years cannot be divided into a whole number of days. While we say that the year is 365 days long, the solar year is actually about 365.2425 days. Thus, if one were to create a calendar of only 365 days, one would lost one day in relation to the solar year every four years. That difference would grow over time until the solar year and the calendar year were many days or weeks different from each other. Obviously, this is an undesirable effect, so calendars that are not solar based (like Stonehenge) must include some way of dealing with that extra fourth of a day ever year if they want to remain accurate.