Julian calendar

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The Julian calendar, proposed by Julius Caesar in 46 BCE (708 AUC), was a reform of the Roman calendar.[1] It was first used in 1 January 45 BCE. It was the main calendar in most of the world, until Pope Gregory XIII replaced that with the Gregorian calendar in 4 October 1582.

During the 20th and 21st centuries, the date according to the Julian calendar is 13 days behind the Gregorian date.

Year length; leap years[change | change source]

The Julian calendar has two types of year: common years of 365 days and leap years of 366 days. There is a simple cycle of three common years followed by a leap year and this pattern repeats forever. However, the rule was not followed in the first years after the of the reform in 45 BCE. Due to a counting error, every 3rd year was a leap year instead of the 4th. The leap years were:[2]

  • 45 BCE (709 AUC)
  • 42 BCE (712 AUC)
  • 39 BCE (715 AUC)
  • 36 BCE (718 AUC)
  • 33 BCE (721 AUC)
  • 30 BCE (724 AUC)
  • 27 BCE (727 AUC)
  • 24 BCE (730 AUC)
  • 21 BCE (733 AUC)
  • 18 BCE (736 AUC)
  • 15 BCE (739 AUC)
  • 12 BCE (742 AUC)
  • 9 BCE (745 AUC)

However, in 8 BCE (746 AUC), emperor Augustus Caesar corrected the problem. The next leap year was 7 CE (160 AUC).

Criticism[change | change source]

With the simple cycle, the length of the Julian year is exactly 365.25 days (365 days and 6 hours), but the actual time it takes for the Earth to go around the Sun once is closer to 365.2422 days (about 365 days, 5 hours, 48 minutes and 46 seconds). This difference is about 365.25 - 365.2422 = 0.0078 days (11 minutes and 14 seconds) each year, although Greek astronomers knew that.[3] This made the seasons get out of track, since the real first day of spring in western Europe (the equinox - day and night the same length) was happening earlier and earlier before the traditional 21 March as the centuries went by. By the 1500s, it was starting around 11 March, ten days 'too early' according to the calendar.

Reform[change | change source]

From Roman calendar[change | change source]

The first step of the reform was to realign the 25 December with the Winter solstice by making 46 BCE (708 AUC) 445 days long. In ordinary Roman calendar, the common year had 355 days and the leap year (one year after the common year) had 378 days. The 46 BCE was a leap year, according to the calendar. Julius Caesar added 67 more days by adding two extra months (those are called Prior and Posterior in letters of Cicero) between November and December.

Sources: [1], [2]
Months 47 BCE
(707 AUC)
46 BCE
(708 AUC)
45 BCE
(709 AUC)
8 BCE
(746 AUC)
January 29 29 31 31
February 28 24 30 28
Intercalaris 27
March 31 31 31 31
April 29 29 30 30
May 31 31 31 31
June 29 29 30 30
Quintilis 31 31 31 31
Sextilis 29 29 30 31
September 29 29 30 30
October 31 31 31 31
November 29 29 30 30
Prior 33
Posterior 34
December 29 29 31 31
Total 355 445 366 365

References[change | change source]

  1. Richards 2013, p. 595.
  2. Michael Douma (2008). Sally Smith (ed.). "The Christian Calendar | Calendars". WebExhibits. Retrieved 2019-05-15.
  3. Claudius Ptolemy, tr. G. J. Toomer, Ptolemy's Almagest, 1998, Princeton University Press, p. 139. Hipparchus stated that the "solar year ... contains 365 days, plus a fraction which is less than by about th of the sum of one day and night".