The Julian calendar is a calendar that was invented by Julius Caesar, the leader of Ancient Rome. It is a reform of the Roman calendar. It was first used in the year 709 AUC (45 BC). Caesar consulted the astronomer Sosigenes of Alexandria. Very probably it was made to approximate the tropical year, which was known at least since Hipparchus.
The calendar has two types of year: regular years of 365 days divided into 12 months and leap years of 366 days. There is a simple cycle of three regular years followed by a leap year and this cycle repeats forever. The Julian year is, therefore, on average 365.25 days long. But in reality, the Earth moves around the Sun about 365.24217 days.
In the 16th century, the Gregorian calendar was introduced in Europe because the Julian Calendar makes too many leap days, which means that it gains a day about every 128 years. The Gregorian calendar has 97 leap days every 400 years, while the Julian had exactly 100. People sometimes use the term Old Style or O.S. to refer to the Julian calendar, with N.S. or New Style referring to the Gregorian calendar. During the 20th and 21st centuries, the O.S. date is 13 days behind the N.S. date.
Table of months[change | change source]
|Months (Roman)||Lengths before 45 BC||Lengths as of 45 BC||Months (English)|
|Februarius||28 (in common years)
In intercalary years:
23 if Intercalaris is variable
23/24 if Intercalaris is fixed
|28 (leap years: 29)||February|
References[change | change source]
- The letter J was not invented until the 16th century.
- The spelling Quinctilis is also used; see page 669 of The Oxford Companion to the Year.