Lunar circle
The moon circle is a term from the Easter calculation . It is synonymous with the Meton cycle and, like this, not clearly used: Both terms can designate both the cyclical series of meetings of the sun and moon in front of the same stars in the sky, which take place every 19 years , as well as the period of 19 years. There is another ambiguity in the Meton cycle because this term is also used without reference to the meeting between the sun and the moon. Then only the summary of 19 years, presumably made by Meton , into the so-called Great or Meton Year is meant.
The circle of the moon in the Julian calendar
For the Easter calculation in the Julian calendar , such a period began when the sun at the vernal equinox (approximating March 21 ) and the moon ( spring full moon ) were in opposition to it. This constellation did not occur in the following 18 years, but it was possible to specify 18 fixed dates in March and April for the spring full moon, which was followed by the sought-after Easter Sunday . In Computus , the medieval algorithm of the Easter calculation, the 19 years of a series were assigned the digits 1 to 19 as the golden number GZ .
The lunar circle in the Gregorian calendar
Just as the Gregorian calendar is not a fundamentally different calendar, but a Julian calendar that has always been used for at least a century, the lunar circle is also used in the Gregorian Easter calculation. 19 Julian calendar years and 235 real lunar months are to a good approximation of the same length (6,939.7500 days against 6,939.6887 days). However, the assumed exact equality had the effect that the Julian Easter calculation was clearly flawed over centuries in the assignment between the calendar beginning of spring and the calendar full moon. In addition, there was an even larger error due to the calendar year being too long in relation to the solar year, which is why both calendar dates lagged more and more behind the events in the sky.
The error of the lunar circle related to the Julian calendar year (365.25 days) is practically eliminated after an average of 312.5 years by moving the calendar spring full moon one (1) day earlier ( lunar equation ). The practically sufficient shortening of the Gregorian calendar year to an average of 365.2425 days must no longer affect the lunar circle, which is why if a Gregorian failure of a leap day (three times in 400 years) occurs, the calendar spring full moon is postponed by one (1) day ( solar equation ) .
Since the Gregorian calendar reform it has become common to use the epact numbers 0 to 29 instead of the golden number to mark the years in the lunar circle. Due to the occasional corrective shift, all calendar days from March 21 to April 19 are eligible for the spring full moon. For at least a century, however, only 19 of these 30 epacts are in use.
See also
Individual evidence
- ^ Heinz Zemanek : Calendar and Chronology, Oldenbourg, 1990, ISBN 3-486-20927-2 , p. 40
- ^ Otto Neugebauer : A History of Ancient Mathematical Astronomie , Springer, 1975, p. 623