The tropical year and the calendar
 

The year used in civil life is based on the tropical year, defined as the interval in time between successive passages of the Sun through the vernal equinox and equalling 365·2422 mean solar days. Because this is not an exact integer, the number of days in a year is made variable so that the seasons do not drift relative to the regular dates as time progresses. 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, excepting those century years (such as 1900 AD) whose numbers of hundreds (in this case 19) are indivisible by four exactly. The year 2000 AD was, therefore, a leap year. 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. On this scheme, the calendar is accurate to 1 day in 3323 years with the result that some particular leap year designated by the rule will need to be abandoned about 3000 years from now.
The presently-used calendar year is the Gregorian, just defined, and introduced by Pope Gregory in 1582. Previously, the Julian calendar had been used in which simply every fourth year was a leap year of 366 days, February 29th being the extra day. This gave an average value for the length of the civil year of 365·25 mean solar days. By 1582, the discrepancy between the number and the length of the tropical year (365·2422 mean solar days) had led to the considerable error of over 12 days. The introduction of the Gregorian calendar removed this error. Unfortunately, political and religious obstacles caused the introduction to be carried out in different countries at different epochs. For example, the change took place in Great Britain in 1752, an act of Parliament in 1751 stating that the year should begin on January 1st (instead of March 25th as had been the custom in England) and that the day following September 2nd, 1752, should be September 14th, thus dropping 11 days. The change in Russia took place even later; the October Revolution of 1917 took place in November, according to countries using the Gregorian calendar.