effect in the year 1582, up to which time the so-called Julian
calendar, as introduced by Julius Caesar, had been everywhere
accepted in Christendom. This Julian calendar, as we have seen,
was a great improvement on preceding ones, but still lacked
something of perfection inasmuch as its theoretical day differed
appreciably from the actual day. In the course of fifteen hundred
years, since the time of Caesar, this defect amounted to a
discrepancy of about eleven days. Pope Gregory proposed to
correct this by omitting ten days from the calendar, which was
done in September, 1582. To prevent similar inaccuracies in the
future, the Gregorian calendar provided that once in four
centuries the additional day to make a leap-year should be
omitted, the date selected for such omission being the last year
of every fourth century. Thus the years 1500, 1900, and 2300,
A.D., would not be leap-years. By this arrangement an approximate
rectification of the calendar was effected, though even this does
not make it absolutely exact.

Such a rectification as this was obviously desirable, but there
was really no necessity for the omission of the ten days from the
calendar. The equinoctial day had shifted so that in the year
1582 it fell on the 10th of March and September. There was no
reason why it should not have remained there. It would greatly
have simplified the task of future historians had Gregory
contented himself with providing for the future stability of the
calendar without making the needless shift in question. We are so
accustomed to think of the 21st of March and 21st of September as
the natural periods of the equinox, that we are likely to forget
that these are purely arbitrary dates for which the 10th might
have been substituted without any inconvenience or inconsistency.