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Ken Ward's Astrology Pages

Astrology: Julian And Gregorian Calendars

clear.gif (807 bytes) Two main calendars have been used in Europe during the last two millennia. They are called the Julian and Gregorian calendars.

Certain historical dates need to be carefully examined to determine which calendar they refer to. For instance, Leonardo da Vinci was born on 15 April 1452, Florentine Calendar, which in Julian is 14 April 1452. The Gregorian (our calendar) date is 23 April 1452. This means, he is a Taurian, and not Aries!

Short History

In 45 BC, Julius Caesar adopted a calendar which was used for hundreds of years afterwards. It is said that he knew that the calendar had some problems, but as they wouldn't occur for hundreds of years, I suppose he didn't appreciate its significance.

However, by the second millennia, certain discrepancies were noted between the seasons and the date. In 1582 Pope Gregory XIII promoted the Gregorian calendar, which was adopted in Catholic Countries soon afterwards. October 4, 1582 was followed by October 15.

Nearly 200 years later, Great Britain adopted this calendar too. Wednesday 2 September 1752 was followed by Thursday 14 September 1752, which lead to riots by people who claimed they had lost 11 days of their lives.

Other countries were even slower to adopt the new calendar, and Russian kept the old calendar until the October (new calendar November) Revolution in 1917.

Historical Dates And Astrology

Astrology programs written in English will most probably add these  missing 11 days for dates before about September 1752. These dates will be wrong for certain European countries which adopted the Gregorian system earlier. Similarly, it will be wrong for countries such as Russia which adopted the system much later. This isn't exactly an error in the programs as it reflects the uncertainty during this period. See Wikipedia. For instance, some people in Sweden were born on 30 February 1712 (A date which does not exist anywhere else in the world!).

All local dates before 1582 will be in the Julian System. Computer programs should translate these correctly.

Certain problems occur:

  1. Has the date already been translated into the Gregorian calendar from a local date?
  2. If a computer program is used, how does it deal with these dates in this country?

Because there are uncertainties, the conclusion is that we need to examine historical dates very carefully, and also be aware of how any computer program will deal with these dates in these countries (latitude and longitude).

Note: Adding 11 days to the Julian date works only for the 1700s, see below. In the 1582s, there were fewer days difference (10 days added). In any case, a whole number of days is always added.

Converting from Julian to Gregorian

The difference in days between Old Style and New Style (for 1752 it was new!) is the number of leap years in the Julian calendar which are not celebrated in the Gregorian calendar. As the 'modern' Julian calendar really started in 300 AD, there are fewer leap year differences than might be expected.

Old Style Year Add Days to dates before 1st March
2000 13
1900 13
1800 12
1700 11
1600 10
1500 10
1400 9
1300 8
1200 7
1100 7
1000 6
900 5
800 4
700 4
600 3
500 2
400 1
300 0 (started First Council of Nicaea in 325)