The Gregorian Calendar
is a Religious Calendar
by Peter Meyer
April 29, 2011

Portuguese translation by Artur Weber & Adelina Domingos

The Gregorian Calendar has its origins in a non-religious calendar invented by Julius Caesar: a structure of months and days (slightly modified by Augustus Caesar) combined with a simple leap year rule.

From before the Nicene Council in 325 CE the Christian church(es) used the Julian Calendar. Years were dated in the Roman manner, either AUC (i.e., from the founding of the City of Rome) or as the number of the year in some Emperor's reign or in some Consul's term of office.

In 525 CE, following a request from one of the Popes, Dionysius Exiguus introduced an epoch for the Julian Calendar with the intention that "Year 1" would designate the year in which Jesus Christ was born, or perhaps Dionysius intended it as the year in which the incarnation occurred. This method of designating years (which method is still in current use) was promoted by the Catholic Church and spread throughout Europe, thereby creating the "Christian" era, which hitherto did not exist. This is the first religious component of (what is now) the Gregorian Calendar.

In Dionysius's chronology years were labelled "Anno Domini", meaning, "in the year of Our Lord (Jesus Christ)". Thus this method of designating years includes an implicit affirmation of the existence of Jesus Christ, who, according to the Nicene Creed, "for us men for our salvation, came down from heaven, and was incarnate by the Holy Spirit of the virgin Mary, and was made man ..." and who now sits "at the right hand of God the Father" in heaven. This is the second religious component of (what is now) the Gregorian Calendar.

At the time of the Nicene Council the various Christian churches of Asia were not all celebrating Easter on the same day, due to divergent methods of fixing the date. In addition to formulating the Nicene Creed (which is the definitive statement of Christian faith) the Nicene Council brought all the churches into line by promulgating a rule for the determination of the date of Easter. This was formulated in terms of the date of the (northern) vernal equinox, specified as March 21, even though in fact the vernal equinox did not always occur on March 21 in the Julian Calendar at that time (so this date is known as the "ecclesiastical vernal equinox" in the context of calculating the date of Easter).

The average length of the year in the Julian Calendar is longer than the vernal equinox year, so by the 16th C. the vernal equinox was occurring on average about 11 days before March 21. This was causing the date of Easter to drift toward summer, clearly inconsistent with the intent of the Nicene Fathers, so the Catholic Church reformed the calendar by (a) dropping 10 days from the calendar and (b) modifying the leap year rule so as to ensure that the average length of the year in the reformed calendar (now called the "Gregorian" calendar) was (almost) the same as the vernal equinox year, thus eliminating the offending slippage. Since (a) and (b) were implemented so as to bring the date of Easter (one of the two main holy days of the Catholic religion) back into accord with the date of the vernal equinox, as intended by the Nicene Fathers, this constitutes the third religious component of the Gregorian Calendar.

Thus although the Gregorian Calendar had its origin over 2000 years ago in a non-religious calendar, it was modified by the Catholic Church in the 6th C. and 16th C. for clearly religious purposes, as described above, and is thus a religious calendar, despite the fact that it is used for non-religious purposes by most people who use it.

A modification of the Gregorian Calendar, which retains its days, months and leap year rule, but which (i) designates years according to the astronomical year numbering system (with years -2, -1, 0, 1, 2, ..., explained in detail at Astronomical Year Numbering and the Common Era Calendar ) and (ii) labels years as "CE" rather than as "BC/AD", may be called "the Common Era Calendar". The acronym "CE" has the virtue that it can be interpreted by Christians as "Christian Era" and by non-Christians as "Common Era", thus allowing both to designate years amiably (from 1 onward) as "CE". Thus non-Christians may use this CE calendar without feeling that in doing so they are implicitly affirming a religious faith which they do not share.

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