Julian and Gregorian Calendars

Julius Caesar realized that a well run empire demanded a uniform and accurate calendar where both past events and future events could be placed on a universally accepted and stable time scale. By his reign, the calendar in use was off by about three months, was based on lunar observations and had been manipulated by Roman bureaucrats to extend their terms in office. This meant that the seasons were off by three months as well. He ordered his Greek astronomer Sosigenes to develop what would become known as the Julian Calendar.

Though it appears that a solar year, the time it takes the Earth to revolve around the sun, is 365 days, it actually takes 365 days, 5 hours, 48 minutes, and 36 seconds (or 365 days plus approximately 6 hours). So the solution appeared simple – just add 24 hours (4 X 6 ), an extra day, every 4 years.

The year 45 BCE (BC) was given 445 days to correct the accumulated errors of the past, and every future year was to have 365 days; and every fourth year was to be designated a leap year and was to have 366 days, adding the extra day to February. But the actual time to revolve around the sun isn't 6 hours but 5 hours, 48 minutes, and 36 seconds and over time, this difference of 11 minutes and 24 seconds adds up.

So this Julian leap year rule created three leap years too many every 385 years which by 1582 caused the Vernal (Spring) Equinox to fall on the 11th day of March instead of the traditional 21st day of March (10 days off). Pope Gregory XIII realized that this meant that the date of Easter would eventually not fall in the spring but would become closer and closer to Dec 25, Christmas. Additionally, Christmas was also drifting off course.

The Pope commissioned astronomer Christopher Clavious to produce a calendar to correct the error. Thus the Gregorian calendar was born. To implement this new calendar, the Pope declared that the day following Thursday, October 4, 1582 was to be Friday, October 15, 1582. Additionally, the Julian leap year rule was modified to assure long range accuracy. Since an extra day every 4 years produced too many leap year days, the rule would not be employed for years ending in 00 (1600, 1700, 1800, 1900, etc.). But this produces too few leap year days so a new rule was made – a year ending in 00 would be a leap year only if it was divisible by 400. Though a great improvement over the Julian calendar, the current leap year rule is still off 1 day every 3300 years.

Since this change was mandated by the Catholic Church, Protestant Europe was reluctant to accept it and remained on the Julian calendar. Germany finally accepted the Gregorian calendar in 1700, 118 years later.
Russia converted 336 years later in 1918 and Greece in 1923. England and her American colonies remained on the Julian calendar until 1752. By this time, the discrepancy between the Julian calendar and the Gregorian calendar had grown to 11 days. To adjust to the Gregorian calendar, September 3, 1752 was changed to September 14, 1752. This prompted riots in which the people yelled "Give us back our eleven days"! George Washington, actually born on February 11, 1732, has had his birthday changed to the currently recognized date of February 22, 1732. Today, we still use the Gregorian calendar and so the year 2000, which is divisible by 400, will be a leap-year.

What about AD and BC ?

In ancient calendars, years were usually numbered according to the year of the ruler's reign. The Catholic Church used the Roman calendar in which year 1 was based on the founding of the city of Rome. A Scythian monk named Dionysius Exiguus wrote a history of the Christian Era called "Cyclus Paschalis" and used the birth of Christ as year 1 for historical events. Using this system, 1 AD would be the year of Christ's birth and 1 BC would be the year previous to 1 AD. Under this system the writing of  "Cyclus Paschalis" would have occurred in 525 AD. This system became the standard in the western world. Emperor Charlemagne (742-814) decreed that this system would be used throughout his empire. During the 16th and 17th centuries, the system was spread throughout the world by the colonial powers. The system has since been de-Christianized by changing BC (before Christ) to BCE (before the common era) and AD (anno Domini)to CE (common era).

Today there are still a number of calendars other than the Gregorian calendar in official use but most of the computer dates used in the conduct of business internationally are Gregorian based.

Millennia

A millennium is a period of 1000 years. The question of which year is the first year of the millennium hinges on the date of the first year AD. Unfortunately the sequence of years going from BC to AD does not include a Year 0.
The sequence of years runs 3 BC, 2 BC, 1 BC, AD 1, AD 2, AD 3 etc. This means that the first year of the first millennium was 1 AD. The one thousandth year was AD 1000 and the first day of the second millennium was AD 1001. So the 20th century runs from 1901 through 2000. The end of the 20th century is December 31, 2000, not December 31, 1999. The start of the new millennium will be 1 Jan 2001.

Baltimore, MD. Tue, 6 Oct 1998
VACETS Column