Calendars: Part 1

A calendar is a system of timekeeping that defines the beginning, length and divisions of the year. In the past, when the calendar had not been invented yet, people could only tell what time of the year it was by the weather. In summer, the days were longer and hotter while in the winter the days were short and cold. Soon people figured out that a year had passed when the sun was in the same point in the sky as it was the previous year.

But humans wanted to measure shorter amounts of time than years, so they began using the moon to measure shorter periods of time. Unfortunately, the time it takes for the moon to go around the earth once (28 days) is not exactly one month.

The calendar (Gregorian calendar) we use today has approximately either 365 or 366 days a year (366 days on leap years). It is divided into 12 months of alternating number of days (30 or 31 except for February which has 28 days or 29 on leap years) that have no relationship to the motion of the moon. In the Gregorian calendar, the tropical year is approximated as 365 97/400 days = 365.2425 days. Thus it takes approximately 3300 years for the tropical year to shift one day in the Gregorian calendar. This approximation is achieved by having 97 leap years every 400 years. An astronomer by the name of John Herschel (1792-1871) thought that a better approximation to the length of the tropical year would be 365 969/4000 days or 365.24225 days. This would mean 969 leap years every 4000 years, rather than the 970 leap years in Gregorian calendar. This could be achieved by dropping one leap year from the Gregorian calendar every 4000 years, which would make years divisible by 4000 non-leap years. This rule, however, has not been used.

references:
Horrible Science: THE TERRIBLE TRUTH ABOUT TIME by Nick Arnold
http://www.google.com.sg/search?hl=en&defl=en&q=define:calendar&sa=X&oi=glossary_definition&ct=title
http://www.webexhibits.org/calendars/index.html
http://en.wikipedia.org/wiki/Pope_Gregory_XIII
http://www.eviple.com/in-heh/img/gregory-xiii.jpg