The Calendar – 24th July 2008

This evening’s main talk, “The History of the Calendar“, was given by Keith Brackenborough. The main purpose of a calendar is to keep track of the days, and make the business of planning, and recording the passage of the years easier. Of course the Earth and Moon do not make this an easy task. The Earth’s most obvious unit of time is the solar day, but it also orbits the Sun in one tropical year, which is, of course, not a whole number of days long. The Moon orbits the Earth, and of course its period is neither a whole number of days nor a whole fraction of a tropical year. Throughout recorded history people have used calendars which took account of the various differences more or less successfully. Keith’s talk covered many of the schemes that have been tried, from the relatively simple Lunar ones to schemes involving cycles many years long. He gave particular emphasis to the development of the calendar used by most of the world today, from its roots in Julius Caesar’s ideas for bringing order to Rome’s chaotic calendar, through Augustus’ tinkering with the lengths of the months, the Council of Nicea’s adjustments for determining the date of Easter, and Pope Gregory’s reforms to account for the slight inaccuracies in Julius Caesar’s original scheme. He told us about the way the Gregorian calendar was adopted slowly over the centuries by more countries, so that it is now the most common calendar, and should be adequate for at least another 2000 years.