Calendars

The Sun appeared to reach the vernal equinox about March 25 in the years immediately after the Roman Empire adopted the Julian calendar. It continued to be the official Roman calendar for the rest of the empire's existence. The Roman Catholic Church adopted the Julian calendar as its official calendar at the Council of Nicaea in A.D.325, soon after the conversion of the emperor Constantine I, who then made Christianity the Roman Empire's official religion. By that time, the Sun was reaching the Vernal equinox about March 21; the fact that the tropical year is 0.0078 day shorter than the average length of the Julian calendar year had accumulated a difference of three to four days from the time when the Julian calendar was first adopted. The Council of Nicaea also renumbered the calendar years; the numbers of the Roman years were replaced by a new numbering system in an effort to have in accord with Christian tradition and beliefs Christ's birth occur in the year A.D.1. (Anno Domini). This effort was somewhat unsuccessful; the best historical evidence indicates that Christ probably was born sometime between 7 B.C. (before Christ) and 4 B.C. Another feature of this modified Julian calendar is that it has no year zero; the 1 B.C. is followed by the year A.D.1.

This Julian calendar remained the official calendar of the Roman Catholic Church for the next 1,250 years. By the year 1575, the Sun was reaching the Vernal Equinox about March 11. This caused concern among both church and secular officials because, if this trend continued, by the year 11,690, Christmas would have become an early spring holiday instead of an early winter one, and would be occurring near Easter.

This prompted Pope Gregory XIII to commission the astronomer Clavius to reform the calendar. Clavius studied the problem, then he made several recommendations. The rate of the precession of the equinoxes was known much more precisely in the time of Clavius than it had been in the time of Sosigenes. The calendar which resulted from the study by Clavius is known as the Gregorian calendar; it was adopted in 1583 in predominantly Roman Catholic countries. It distinguished between century years, that is, years such as 1600, 1700, 1800, 1900, 2000, etc., and all other years, which are non-century years. The Gregorian calendar has the following main features:

All non-century years evenly divisible by four, such as 1988, 1992, and 1996 are leap years with February 29th as the 366th day. All other non-century years are ordinary years with 365 days.

Only century years evenly divisible by 400 are leap years; all other century years are ordinary years. Thus, 1600 and 2000 were leap years with 366 days, while 1700, 1800, and 1900 had only 365 days.

The Gregorian calendar was reset so that the Sun reaches the Vernal Equinox about March 21. To accomplish this, ten days were dropped from the Julian calendar; in the year 1582 in the Gregorian calendar, October 4 was followed by October 15.

The Gregorian calendar is the official calendar of the modern world. From the rules for the Gregorian calendar shown above, one finds that, in any 400-year interval, there are 97 leap years and 303 ordinary years, and the average length of the Gregorian calendar year is 365.2425 days. This is only 0.0003 day longer than the tropical year. This will lead to a discrepancy of a day in about the year 5000. Therefore, the Sun has usually reached the Vernal Equinox and northern hemisphere Spring has begun about March 21 according to the Gregorian calendar.

The Gregorian calendar was not immediately adopted beyond the Catholic countries. For example, the British Empire (including the American colonies) did not adopt the Gregorian calendar until 1752, when 11 days had to be dropped from the Julian calendar, and the conversion to the Gregorian calendar did not occur in Russia until 1917, when 13 days had to be dropped.

One feature of the Gregorian calendar is that February is the shortest month (with 28 or 29 days), while the summer months July and August have 31 days each. This disparity becomes understandable when one learns that Earth's orbit is slightly elliptical with eccentricity 0.0167, and the earth is closest to the Sun (at perihelion) in early January, while it is most distant from the Sun (at aphelion) in early July. It follows from Kepler's Second Law that Earth, moving fastest in its orbit at perihelion and slowest at aphelion, causes the Sun to seem to move fastest on the ecliptic in January and slowest in July. The fact that the Gregorian calendar months January, February, and March have 89 or 90 days, while July, August, and September have 92 days makes some allowance for this.