An introduction to astronomy: designed as a text-book for the use of students in college. By Denison Olmsted ...

THE CALENDAR. 45
1 1 1. The most ancient nations determined the number of
days in the year by means of the stylus, a perpendicular rod
which cast its shadow on a smooth plane, bearing a meridian
line. The time when the shadow was shortest, would indicate
the day of the summer solstice; and the number of days
which elapsed until the shadow returned to the same length
again, would show the number of days in the year. This was
found to be 365 whole days, and accordingly this period was
adopted for the civil year. Such a difference, however, between the civil and astronomical years, at length threw all
dates into confusion. For, if at first the summer solstice happened on the 21st of June, at the end of four years, the sun
would not have reached the solstice until the 22d of June,
that is, it would have been behind its time. At the end of
the next four years the solstice would fall on the 23d; and in
process of time it would fall successively on every day of the
year. The same would be true of any other fixed date.
Julius Caesar made the first correction of the calendar, by introducing an intercalary day every fourth year, making February to consist of 29 instead of 28 days, and of course the
whole year to consist of 366 days. This fourth year was denominated Bissextile.* It is also called Leap Year.
11 2. But the true correction was not 6 hours, but 5h.
49m.; hence the intercalation was too great by 11 minutes.
This small fraction would amount in 100 years to 4 of a day,
and in 1000 years to more than 7 days. From the year 325 to
1582, it had in fact amounted to about 10 days; for it was
known that in 325, the vernal equinox fell on the 21st of
Mlarch, whereas, in 1582 it fell on the 11th. In order to restore the equinox to the same date, Pope Gregory XIII.
decreed that the year should be brought forward ten days, by
reckoning the 5th of October the 15th. In order to prevent
the calendar from falling into confusion afterward, the following rule was adopted:
Every year whose ~nu.mber is not divisible by 4 without a renainder, consists of 365 days; every year which is so divisible,
but is not divisible by 100, of 366; every year divisible by 100
n \xle sextus dies ante Kalendas'beirag eckouedA twice (Bis).