The year used in civil life is based on the *tropical year*, defined
as the interval of time between successive passages of the Sun through the
first
point of Aries. Because of
precession, the first point of Aries moves along the ecliptic at an
average rate of about 50 arcseconds per year in the direction opposite
to that in which the Sun moves. The tropical year, of length 365.2422
mean solar days, is
therefore shorter than
the *sidereal year*, of length 365.2564 mean solar days, defined to
be the time it takes the Sun to make one complete circuit of the ecliptic.

For convenience, the calendar year contains an integral number of days,
either 365 or 366. Every fourth year, called a *leap year*, has
366 days, February 29th being the extra day, excepting those century
years (such as 1900 AD) which are indivisible by 400 exactly. These
rules give a mean civil year equal in
length to 365.2425 mean solar days, a figure very close to the number of
mean solar days in a tropical year.

The calendar described above was introduced by Pope Gregory in 1582 and is
known as the *Gregorian* calendar. Previously, the *Julian*
calendar had been used in which *every* fourth year was a leap year of 366 days.
This gave an average value for the length of the civil year of 365.25 mean
solar days. By 1582, the discrepancy between this number and the length
of the tropical year had led to the considerable error of over 12 days. The
introduction of the Gregorian calendar removed this error, although not all
countries introduced it at the same time (e.g. the change did not take place
in Great Britain until 1752).

The irregularities in the present calendar (unequal months, days of
the week having different dates from year to year) and the changes
from the Julian to the Gregorian calendar make it difficult to compare
the lengths of time between observations made many years apart. The
*Julian Date* system was therefore introduced to reduce the
computational labour in determining historical time intervals. Noon
on January 1st 4713 BC was chosen as the starting point, time being
measured from that epoch by the number of days that have elapsed since
then. For example, the Julian Date for midnight on September 22nd,
2006 was 2 454 000.5.

*©Vik Dhillon, 30th September 2009*