THE CALENDAR AND LEAP YEAR 2008
by Bob Hayward, Astronomer/Educator

It’s leap year once again, something that happens every four years, every year evenly divisible by four, unless, of course, it is a century year, one evenly divisible by 100, unless, of course, that century year is evenly divisible by 400. Okay! Got that? Those are the rules for leap year under the Gregorian Calendar. Where did all that confusion come from?

Well, let’s start with a fact of nature, one we have known about for many centuries. The Earth revolves in its orbit around the Sun once every 365.2422 days. Astronomer call this the Tropical Year. It was important to the ancient peoples, living in an agrarian society to keep track of the events that had effects on their crops. They needed to know when the seasons were beginning and ending so they would know when to plants certain crops and when to expect to harvest them. In ancient Egypt it was critically important to know when the River Nile was going to flood and enrich the land with its topsoil. So, calendars were important.

The ancients also were astute observers of the sky. They noticed the cyclical patterns of objects and events in the sky most notably the Moon and the Sun. The Babylonians used the Moon to measure their month. However, since the Moon goes through its phases every 29½ days, a year of 12 months comes to 12 x 29½ = 354 days. The calendar very rapidly fell out of sync with the seasons.

The ancient Egyptians timed their year by the reappearance of the start Sothis whose rising in the early morning, called heliacal rising by astronomers, signaled the coming flood of the Nile. They noted the year was 365 days long. They also realized there was an extra quarter day to be added but they never had that implemented. (And we think we have problems getting Congress to act!)

The Romans originally had a calendar named after gods and numbers. We still have remnants of that today. September, which comes from the Latin for seven, is the seventh month; October, after the Latin number eight, is the eighth month, etc. They then added January and February to the end of their calendar making it a year of 12 months. Clear?

By 47 BC spring was occurring in June! Julius Caesar obtained the services of the astronomer Sosigines in Alexandria who came up with a plan. 46 BC had 445 days! Then the calendar had 12 months of either 31 or 30 days with 29 in February, the last month of the year. Every fourth year February had 30 days. Since Julius Caesar didn’t have to deal with a Congress, this so-called Julian Calendar was implemented. After his death the fifth month, Qunitilius, was renamed Julius in his honor. Following Julius Caesar along came Augustus Caesar. Now, he couldn’t have a month named after Julius and not himself; so he changed the name of the sixth month from Sextilius to Augustus. Not only that, Julius had 31 days and Augustus only 30; so he took another day from poor February, to make his month as long as Julius.

Now, fast forward to 1563. This is the year that King Charles IX of France decided that the year should begin near the celebration of the birth of Christ. So he changed the beginning of the year from March to January. Now, September, named after the Latin number seven, became the ninth month, October, named after number eight became month number ten, etc. At least, this was true in the Roman Catholic countries.

With an extra day added for a leap year every fourth year, the average length of the calendar was 365.2500 days long. That’s a whopping error of 0.0078 days per year. Wow! By 1582 this amounted to an error of ten whole days. Recall that the church calculates the date of Easter based on the first day of spring, the vernal equinox. So, this was unacceptable. After forming a committee to study the situation, Pope Gregory XIII set up by decree what we now know as the Gregorian Calendar. Here are his rules:

• October 4, 1582 was followed by October 15. There were riots when people thought the Pope had cut ten days from their lives. Workers wanted to be paid for the ten missing days and landlords wanted rent for them.
• New rules for leap year:
  • Years evenly divisible by 4 are leap years unless they are century years, i.e., years evenly divisible by 100.
  • Century years are not leap years unless they are evenly divisible by 400.
  • Years evenly divisible by 400 are leap years.
  Thus, 1900 was not a leap year but 2000 was. And 2008 is a leap year since it is evenly divisible by 4 and is not a century year.

Now what all this did was to make the average length of the year equal to 365.2425 days, and error of 0.0003 day per year or and one day every 3300 years. So, the next time we have to worry about the calendar being off by one day from the seasons will be in the year 1582 + 3300 = 4882. We can wait!

Oh, yes, a footnote to all this. In 1582 the Protestant countries did not go along with Pope Gregory’s decree nor had they changed the beginning of the year to January 1. In England the year began on March 25. It wasn’t until September 2, 1752 that those countries, including the American colonies, changed from the ancient Julian calendar to the Gregorian calendar. By then the calendar was out of sync with the seasons by 11 days. George Washington was born February 22, 1732 by the Gregorian calendar and we used to celebrate his birthday on that day. But if you had been present and had checked the calendar on the wall, it would have read February 11 since Virginia was still using the Julian calendar in 1732. All of this is, of course, perfectly clear to your family genealogist who has to deal with the calendar change when working with ancestral data before 1732. Thus, you sometimes see double dates indicating the old and new calendar dates.

By the way, some have suggested the whole world move to the so-called World Calendar. It has various forms but one of the best known would have four identical quarters, each beginning on a Sunday, having months of 31 + 30 + 30 days for a total of 91 days. This totals to 364 days so there would be and extra World Day following December 30 and preceding January 1. It would be a worldwide holiday and would not count as a day of any week. Each leap year, under the same rules as for the Gregorian calendar, there would be a Leap Day between June 30 and July 1. It would also be a worldwide holiday. With an identical calendar for every year, in fact, for every quarter, holidays, birthday, important meetings and celebrations, etc, would all occur on the same day of the week every year. Would that be cool, or what!