The Julian calendar, or as formerly transliterated from French into the Julian calendar, was introduced by Julius Caesar in 46 BC and was in effect from 45 BC (709 ab urbe condita). It was selected after consultation with the Alexandrian astronomer Sosigenes and was designed to approximate the tropical year, which has been known since at least the time of Hipparchus. It has 365-day common years divided into 12 months, and leap days are added to February every 4 years. So the average Julian year is 365.25 days long.
The Julian calendar remained in use until the 20th century in some countries and is still used by some Orthodox churches. However, too many leap days are added to match the sidereal seasons in this scheme. On average, the equinoxes and solstices occur about 11 minutes a year earlier than the Julian calendar, giving it an extra day every 134 years. While Hipparchus and perhaps Sosigenes were aware of this difference (though possibly not the exact value), it was considered of little importance. However, it accumulated significantly over time and eventually led to the reform of the calendar in 1582 in which the Julian calendar was replaced with the more accurate Gregorian calendar.
The notation "old style" (OS) is sometimes used to indicate dates in the Julian calendar, as opposed to "new style" which is used to indicate dates in the Gregorian calendar. This notation is used when there is a danger of confusing date figures in documents.
From Rome to Julius
The regular year in the earlier Roman calendar contained 12 months totaling 355 days. To supplement, the leap month, Mensis Intercalaris, is sometimes inserted between February and March. This leap month is created by inserting 22 days before the last 5 days of February, creating a 27-day month. It begins when February is truncated to 23 or 24 days, so it adds 22 or 23 days to the year, creating a leap year of 377 or 378 days.
According to Censorinus and Macrobius, the ideal leap cycle consists of ordinary years with 355 days interspersed with leap years, which alternate 377 and 378 days. In this system, the Roman year averaged 366.25 days over four years, making it an average shift of about 1 day per year when compared to any solstice or equinox. Macrobius describes a further improvement, in which there are 8 years out of 24 with only 3 leap years having 377 days and 5 common years 355 days (the remaining 16 years are 8 ordinary years, 4 leap years have 377 days and 377 days). 4 leap years 378 days). This completion makes the average length of the year 365.25 days over 24 years. In practice, leap years do not take place in a scheme that conforms to these ideal systems, but as determined by the popes. As can be determined from the historical evidence, they have not been as regular as the ideal models suggest. They usually occur in the second or third year of a four-year cycle, but are sometimes neglected for longer, and sometimes two years in a row.
If this system is properly managed, on average the Roman year will be quite consistent with the tropical year. However, if too many leap years