Julian Calendar
The Julian calendar was adopted in Rome in 45 B.C., taking over from the Macrobius calendar, which was full of errors and did not tend to follow tropical years if intercalary years were neglected or miscalculated. Intercalary years consisted of the addition of a month called Mensis Intercalais, which consisted of either 22 or 23 days, occurring between modern February and March. The Julian calendar, which is a precursor to the modern Gregorian calendar, consists of twelve months of varying length. The number of days in each month differs from the modern Gregorian calendar, though the names of the months tend to align, as the Gregorian calendar is a refinement of the Julian calendar.
Unlike the Macrobius calendar, the Julius Calendar was more stable, introducing the concept of leap years to account for the time it takes for the earth to circle the sun. However, the method of calculation for leap years was in error, resulting in too frequent leap years. Instead of these years occurring every four years, they happened every three years. This resulted in a shift of the natural seasons in comparison to the calendar, which was corrected upon the adoption of the Gregorian calendar.
Unlike many calendars, the Julian calendar renamed months. This practice was to honor the rulers of the time, although only two changes remained permanent. The original months were named Ianuarius, Februarius, Martius, Aprilis, Maius, Iunius, Quintilis, Sextilis, September, October, November, and December. Quintilis and Sextilis were renamed to July and August honor of Julius Caesar and Augustus. Quintilis was renamed after Julius Caesar, as this was the year of his birth. Sextilis was renamed as major events in Augustus' life occurred during what would become known as August.
After the establishment of the Julian calendar, many countries began shifting to other calendar systems, which caused confusion for the few who did travel. The Julian calendar saw most use in Europe and North Africa, with the rest of the world using their own calendar systems. It was not until 1582 when Pope Gregory XIII established the Gregorian calendar that the world slowly shifted to universally using a single calendar.
However, the Julian calendar has not faded completely from use. It is still adopted for solar purposes by some orthodox churches, and it is currently in use for agricultural purposes in North Africa. Due to the alterations in leap years, the Julian calendar differs from the Gregorian calendar by 13 days. In 2010, it will differ by 14.