Why were there only 19 days in September 1752 in Great Britain but 30 in Spain? Why is this year a leap year when 1800 and 1900 weren't? And for that matter, why hath September, April, June, and November 30 days?
The Gregorian Calendar Explained: History and Mathematics, a lecture by Temple University Math Professor Yury Grabovsky, held Tuesday, Feb. 29 (leap day), in Room 103, of the Tuttleman Learning Center, 13th St. and Montgomery Ave., will answer these questions and more.
"Leap year 2000 closes the first ever 400-year cycle in the history of our calendar and I wanted to do something to celebrate this rare event," says Grabovsky.
The lecture will trace the evolution from early calendars, which were based on the phases of the moon, to our modern solar year-based calendar, says Grabovsky.
While many changes in the calendar were made to increase its accuracy, just as many were made for political and religious reasons, he says.
A prime example of an arbitrary change in the calendar is the short month of February, which fell victim to the vanity of the Roman emperors Julius and Augustus Caesar.
"When the emperors named the months of July and August after themselves they each took a day from poor February," says Grabovsky.
And even when scientists noted that the calendar was dramatically incorrect, it took political backing for change to take place, he notes.
"As early as 1000 A.D., there were suggestions to change the Julian calendar but to reform a calendar is a tremendous political decision. While astronomers knew it was inaccurate, no one was able to get the political backing to make a change," says Grabovsky.
The calendar continued to become more grossly inaccurate until 1582, when, by decree of Pope Gregory XIII, it was adjusted.
That year, Gregory shortened the month of October by 10 days and decreed that only centennial years that are divisible by 400 would be leap years.
And while these changes made the calendar much more accurate, more than anything else, the overriding impetus for the change was to correct the date on which Easter would fall, notes Grabovsky.
Protestant countries, led by Great Britain, didn't make the change until September 1752, by which time their calendar was off by 11 days.
Yet with all the ulterior motives for the changes that have been made in the calendar over the years, the Gregorian calendar turns out to be an incredibly accurate way of tracking time, says Grabovsky.
"It's one of the simplest and yet most precise systems you can devise for tracking something as imprecise as the length of the year.
"It has to do with a mysterious and beautiful mathematical object called the continued fraction," says Grabovsky.
During the lecture Grabovsky will show just how accurate the calendar is by using continued fractions, which give the closest approximation possible to an irrational number, such as the length of the year.
"It turns out we have the second best possible calendar that combines simplicity and precision," he says.
He'll also demonstrate how far off our calendar will one day be under the current system and what may be done to correct it.
mg-477 February 24, 2000