This article is from the Calendars FAQ, by Claus Tondering firstname.lastname@example.org with numerous contributions by others.
The Julian calendar was introduced by Julius Caesar in 45 BC. It was
in common use until the 1500s, when countries started changing to the
Gregorian calendar (section 2.2). However, some countries (for
example, Greece and Russia) used it into the 1900s, and the Orthodox
church in Russia still uses it, as do some other Orthodox churches.
In the Julian calendar, the tropical year is approximated as 365 1/4
days = 365.25 days. This gives an error of 1 day in approximately 128
The approximation 365 1/4 is achieved by having 1 leap year every 4