CALENDAR, Gregorian, 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. Therefore, the Gregorian calendar differs from the Julian, both in the form of the year, and in that epacts are substituted instead of golden numbers. See EPACT.
Dr. Playfair, in his " System of Chro nology," observes, that the method of in tercalation used in the Gregorian Calen dar is not the most accurate. Ninety-se ven days, or 100-3, are inserted in the space of four centuries. This supposes the tropical year to consist of 365d, 5h, 49', 12". On this supposition the inter polation would be exact, and the error would scarcely exceed one day in 268,000 years. But the reformers of the calen dar made use of the Copernican year of 365d, 5t.49', 20". Instead, therefore, of inserting 97 days in 400 years, they ought to have added, at proper intervals, 41 days in 169 years, or 90 days in 371 years, or 131 in 540 years, &c. Recent obser vations have determined the quantity of the tropical year to be 365d, 5h, 48', 45i".
Admitting this to be the true quantity of it, the intercalations ought to be made as follows: — — — — 1813 1441 2754 4067 9447 51302 318' 349' 667' 985' 2288' 12425' 60749 172800 ; that is, one day ought to 14713, 41351 be intercalated in the space of 4 years, or rather 4 days in 17 years, or 8 days in 33 years, &c. If 41,851 days were inter calated in 172,800 years, there would be no error. The signs and — indicate that the number of intercalary days above which they are placed is too great or too small. Every succeeding number is more accurate than that which goes before. As this method of interpolation is different from that now in use, it is obvious that the Gregorian calendar must be corrected after a certain period of years. The cor rection, however, will be inconsiderable for many ages, as it will amount only to a day and a half, which is to be suppressed in the space of 5000 years.