The Julian calendar is a solar calendar of 365 days in every year with an additional leap day every fourth year (without exception). The Julian calendar is still used in parts of the Eastern Orthodox Church and in parts of Oriental Orthodoxy as well as by the Berbers, whereas the Gregorian calendar is used in most parts of the world.
This calendar, proposed by Roman consul Julius Caesar in 46 BC, was a reform of the earlier Roman calendar, a largely lunisolar one. It took effect on 1 January 45 BC, by edict. It was designed with the aid of Greek mathematicians and astronomers such as Sosigenes of Alexandria.
The calendar became the predominant calendar in the Roman Empire and subsequently most of the Western world for more than 1,600 years until 1582, when Pope Gregory XIII promulgated a minor modification to reduce the average length of the year from 365.25 days to 365.2425 days and thus corrected the Julian calendar's drift against the solar year.