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Next: 0.5.5.2 Source Code Up: 0.5.5 Julian Calendar Algorithms Previous: 0.5.5 Julian Calendar Algorithms

0.5.5.1 A Brief History of the Julian Calendar

The original Julian Calendar was introduced by Julius Caesar (thus the name) in 44 B.C. The length of a single year was to be equal to the time time it takes for the earth to orbit the sun once. This amount was estimated to be 365 days and 6 hours. Every fourth year the extra six hours were collected and added as an extra day to the year, creating a leap year of 366 days.

Because of a slight inaccuracy in estimation of a year's length under the original Julian Calendar (The true amount of time it takes for the Earth to complete one solar orbit is closer to 365 days, 5 hours, 49 minutes and 12 seconds), over a long period of time the extra minutes and seconds began to accumulate and things began to go wrong. Most notably the Spring Equinox, which occurs at a measurable fixed point in the Earth's orbit, began not to fall on the same date.

The ``new style'' Julian Calendar, upon which the algorithm presented here is based, dates from the year 1582. That is when Pope Gergory XIII decided to fix the messed up Julian calendar. To do so he skipped the date forward from October 5th to October 14th to trim time added due to the inaccuracy of the Julian calendar. He also redefined which years would be counter as leap years. Before, every year evenly divisible by four was leap; Pope Gregory changed the rules so that: All years evenly divisible by four were leap years except those evenly divisible by 100 and not also evenly divisible by 400. For instance, 1700, 1800, and 1900 are not leap years. They are evenly divisible by four but are also divisible by 100. The year 2000 is a leap year because it is evenly divisible by four and, even though it is also divisible by 100, it is also divisible by 400. The Gregorian calendar also set New Year's day to January first. The ``new style'' Julian Calendar is popularly called the Gregorian Calendar in honor of Pope Gregory.

Interestingly, only Catholic countries used this better calendar right away. Many Protestent countries, however, adopted it over the next two centuries. Most did so around 1700. England held out until 1752. Others waited until the early 1900s. Some Greek Orthadox countries opted to create their own calendar (a modified version of the Gregorian system) and use it instead today.

The ``new style'' (Gregorian) calendar should accurately model the Earth's movement around the sun for some 40,000 more years.

A mistake commonly made is to confuse ``Julian day numbers'' with the ``Julian calendar.'' The Julian calendar is the system implemented in 44 A.D. by Caesar. Julian day numbers (which are based on the calendar and are calculated by the code in the following section) were proposed by a monk and named ``Julian'' after his father (not Caesar). Day numbers are simply the number of days elapsed since a fixed starting date. That date is arbitrary but often set to January 1st, 4713 B.C. (which the monk thought was pretty darn close to when the biblical creation story in Genesis must have taken place). Other day numbering routines use different days as ``day zero.''

Scott Gasch
1999-07-09