aving been discovered, Augustus ordered that all the years
from the thirty-seventh of the era to the forty-eighth inclusive should be
common years, by which means the intercalations were reduced to the proper
number of twelve in forty-eight years. No account is taken of this blunder
in chronology; and it is tacitly supposed that the calendar has been
correctly followed from its commencement.

Although the Julian method of intercalation is perhaps the most convenient
that could be adopted, yet, as it supposes the year too long by 11 minutes
14 seconds, 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 year and the equinox.
Sosigenes could scarcely fail to know that this year was too long; for it
had been shown long before, by the observations of Hipparchus, that the
excess of 3651/4 days above a true solar year would amount to a day in 300
years. The real error is indeed more than double of this, and amounts to a
day in 128 years; but in the time of Caesar the length of the year was an
astronomical element not very well determined. In the course of a few
centuries, however, the equinox sensibly retrograded towards the beginning
of the year. When the Julian calendar was introduced, the equinox fell on
the 25th of March. At the time of the council of Nice, 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 equinox to
its former place, Pope Gregory XIII. 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 years, he ordered the intercalations to be
omitted on all the centenary years excepting those which are multiples of
400. According to the Gregorian rule of intercalation, therefore, every
year of which the number is divisible by four without a remainder is a leap
year, excepting the centurial years, which are only leap years when
divisible by four after omitting the two ciphers. Thus 1600 was a leap
year, but 1700, 1800 and 1900 are common years; 2000 will be a leap year,
and so on.

As the Gregorian method of intercalation has been adopted in all Christian
countries, Russia excepted, it becomes interesting to examine with what
degrees of accuracy it reconciles the civil with the solar year. According