By Earl Hunsinger
In 1582, millions in Catholic countries went to sleep on October 4th and didn't wake until October 15th. They knew beforehand that this would happen. The Pope had ordered it. Upon counsel from his advisors, he had decreed that October 5th to 14th were to be skipped that year. Why did he do such a seemingly strange thing? To understand, it is necessary to understand something about the calendar that we use.
Man has always kept track of time. The passing of the seasons is obvious, and has always been important to farmers. With a few simple observations, it also becomes obvious that the sun and the moon mark the passage of time. The apparent movement of the sun is related to the seasons. This regular cycle of seasons as the earth circles the sun designates a year.
In the same way, the moon as it passed through its various phases came to be used to keep track of shorter periods. These periods are known as lunar months, or just months. In fact, the word month actually comes from the Old English word for moon. There are approximately 12 lunar months in a solar year. Since the moon takes 29.53059 days to go through its cycle, the months we use today no longer correspond to the lunar months used by ancient people. This means that it is now possible to have two full moons in a month. As the website Pulse Planet points out, the second full moon in a month is sometimes known as a blue moon, which according to some, is the derivation of the expression "once in a blue moon."
Today we don't worry about the cycles of the moon. The word month is just an arbitrary term for a period of 30 or 31 days. We know that there are twelve of these months in a year. We know that there are 365 days in a year, with each season coming around a certain month each year. Oh yes, and we may remember that every fourth year is a leap year, where we add a day in February. There is a minor curiosity, and then perhaps only if we happen to know someone born on February 29th.
The world today has, for the most part, standardized on the same calendar. In earlier times, people weren't so fortunate. The calendar we use is a modification of a calendar introduced by Julius Caesar in 45 B.C. and sometimes called the Julian calendar. He introduced the idea of a leap year, where an extra day would be added every fourth year. This was needed because the earth doesn't actually go around the sun in an even 365 days. The time taken is closer to 365 ¼ days.
The year is actually 365.242199 days long. This means that by 1582, the calendar was off by 10 days. To correct this, Pope Gregory XIII ordered that 10 days in October be eliminated. He also introduced a method to ensure that this problem didn't happen again. Every 100 years, like years 1700 and 1800, unless they are evenly divisible by four, such as 1600 or 2000 will only be leap years. The calendar we use today is known as the Gregorian calendar in his honor.
As the Simon Fraser University's 'History of Mathematics' webpage explains, the change from the old style or Julian calendar to the new style or Gregorian calendar did not take place everywhere at once. The break between the Eastern and Western Church, and between Protestants and Catholics, caused some resistance to following any suggestion given by the Catholic Pope. An example being, Britain and her colonies, including those in America, didn't change over to the new style calendar until September of 1752, by which time it was necessary to skip 11 days instead of 10. China didn't change until 1949.
Of course, even using the Gregorian calendar, the seasons will drift a little over the millennia. To make it more accurate, some in recent years have suggested adding an additional rule: years evenly divisible by 4,000 will not be leap years. This would make the calendar accurate to one day in about 20,000 years.