Page:Encyclopædia Britannica, Ninth Edition, v. 4.djvu/739

.] was inserted in the calendar between the 24th and 25th day. having then twenty-nine days, the 25th was the 6th of the of, sexto calendas; the preceding, which was the additional or intercalary day, was called bis-sexto calendas,—hence the term , which is still employed to distinguish the  of 366 days. The denomination of  would have been more appropriate if that  had differed from common years in defect, and contained only 364 days. In the ecclesiastical calendar the intercalary day is still placed between the 24th and 25th of ; in the civil calendar it is the 29th. The regulations of were not at first sufficiently understood; and the s, by every third  instead of every fourth, at the end of thirty-six s had intercalated twelve times, instead of nine. This mistake having been discovered, ordered that all the s from the thirty-seventh of the era to the forty- eighth inclusive should be s, by which means the intercalations were reduced to the proper number of twelve in forty-eight s. No account is taken of this blunder in ; and it is tacitly supposed that the calendar has been correctly followed from its commencement.

Although the Julian method of is perhaps the most convenient that could be adopted, yet, as it supposes the too long by 11 s 14 s, it could not without correction very long answer the purpose for which it was devised, namely, that of preserving always the same interval of time between the commencement of the  and the. could scarcely fail to know that this was too long; for it had been shown long before, by the observations of, that the excess of 365$1⁄4$ days above a true solar  would amount to a day in 300 s. The real error is indeed more than double of this, and amounts to a day in 128 s; but in the time of  the length of the  was an  element not very well determined. In the course of a few centuries, however, the sensibly retrograded towards the beginning of the. When the Julian calendar was introduced, the fell on the 25th of. At the time of the of, which was held in 325, it fell on the 21st; and when the reformation of the calendar was made in 1582, it had retrograded to the 11th. In order to restore the to its former place,   directed ten days to be suppressed in the calendar; and as the error of the Julian intercalation was now found to amount to three days in 400 s, he ordered the s to be omitted on all the centenary s excepting those which are multiples of 400. According to the rule of, therefore, every  of which the number is divisible by four without a remainder, is a leap , excepting the centurial s, which are only leap s when divisible by four after omitting the two s. Thus 1600 was a leap , but 1700, 1800, and 1900 are s; 2000 will be a leap , and so on.

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