Gregorian KALENDAR, is that which, by means of epacts, rightly disposed through the several months, determines the new and full moons, and the time of Easter, with the moveable feasts depending thereon, in the Gregorian year.
The Gregorian kalendar, therefore, differs from the Julian, both in the form of the year, and in that epacts are substituted in lieu of golden numbers: for the use and disposition whereof, see EPACT.
Though the Gregorian kalendar be preferable to the Julian, yet it is not without its defects (perhaps as Tycho Brahe and Cassini imagine, it is impossible ever to bring the thing to a perfect justness). For, first, The Gregorian intercalation does not hinder, but that the equinox sometimes succeeds the 21st of March as far as the 23d; and sometimes anticipates it, falling on the 19th; and the full moon, which falls on the 20th of March, is sometimes the paschal; yet not so accounted by the Gregorians. On the other hand, the Gregorians account the full moon of the 22d of March the paschal; which yet falling before the equinox, is not paschal. In the first case, therefore, Easter is celebrated in an irregular month; in the latter, there are two Easters in the same ecclesiastical year. In like manner, the cyclical computation being founded on mean full moons, which yet may precede or follow the true one by some hours, the paschal full moon may fall on Saturday, which is yet referred by the cycle to Sunday: whence, in the first case, Easter is celebrated eight days later than it should be; in the other, it is celebrated on the very day of the full moon, with the Jews and Quartodeciman heretics; contrary to the decree of the council of Nice. Scaliger and Calvinus show other faults in the Gregorian kalendar, arising from the negligence and inadvertency of the authors; yet is this kalendar adhered to by the Romans throughout Europe, &c. and used wherever the Roman breviary is used.
Reformed or Corrected KALENDAR, is that which, setting aside all apparatus of golden numbers, epacts, and dominical letters, determines the equinox, with the paschal full moon, and the moveable feasts depending thereon, by astronomical computation, according to the Rudolphine Tables.
This kalendar was introduced among the Protestant states of Germany in the year 1700, when 11 days were at once thrown out of the month of February; so that in 1700 February had but 18 days: by this means, the corrected style agrees with the Gregorian. This alteration in the form of the year they admitted for a time; in expectation that, the real quantity of the tropical year being at length more accurately determined by observation, the Romanists would agree with them on some more convenient intercalation.
Construction of a KALENDAR or Almanack. 1. Compute the sun's and moon's place for each day of the year; or take them from ephemerides. 2. Find the dominical letter, and by means thereof distribute the ka-
lendar into weeks. 3. Compute the time of Easter, and thence fix the other moveable feasts. 4. Add the immovable feasts, with the names of the martyrs. 5. To every day add the sun's and moon's place, with the rising and setting of each luminary; the length of day and night; the crepuscula, and the aspects of the planets. 6. Add in the proper places the chief phases of the moon, and the sun's entrance into the cardinal points; i. e. the solstices and equinoxes; together with the rising and the setting, especially heliacal, of the planets and chief fixed stars. See ASTRONOMY.
The duration of the crepuscula, or the end of the evening and beginning of the morning twilight, together with the sun's rising and setting, and the length of days, may be transferred from the calendars of one year into those of another; the differences in the several years being too small to be of any consideration in civil life.
Hence it appears, that the construction of a kalendar has nothing in it of mystery or difficulty, if tables of the heavenly motions be at hand.
Some divide calendars or almanacks into public and private, perfect and imperfect; others into Heathen and Christian.
Public almanacks are those of a larger size, usually hung up for common or family use; private are those of a smaller kind, to be carried about either in the hand, inscribed on a staff, or in the pocket; perfect, those which have the dominical letters as well as primes and feasts inscribed on them; imperfect, those which have only the primes and immovable feasts. Till about the fourth century, they all carry the marks of heathenism; from that age to the seventh, they are generally divided between heathenism and Christianity.
Almanacks are of somewhat different composition, some containing more points, others fewer. The essential part is the kalendar of months and days, with the rising and setting of the sun, age of the moon, &c. To these are added various parerga, astronomical, astrological, meteorological, chronological, and even political, rural, medical, &c. as calculations and accounts of eclipses, solar ingresses, aspects, and configurations of the heavenly bodies, lunations, heliocentric and geocentric motions of the planets, prognostics of the weather, and predictions of other events, tables of the planetary motions, the tides, terms, interest, twilight, equation, kings, &c.
Gelalean, or Jellalwan KALENDAR, is a correction of the Persian kalendar, made by order of Sultan Gelaleddan, in the 467th year of the Hegira; of Christ 1089.