# Julian calendar

calendar introduced by Julius Caesar in 45 BC

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.

 Gregorian calendar 2023MMXXIII Ab urbe condita 2776 Armenian calendar 1472ԹՎ ՌՆՀԲ Assyrian calendar 6773 Bahá'í calendar 179–180 Balinese saka calendar 1944–1945 Bengali calendar 1430 Berber calendar 2973 British Regnal year 71 Eliz. 2 – 72 Eliz. 2 Buddhist calendar 2567 Burmese calendar 1385 Byzantine calendar 7531–7532 Chinese calendar 壬寅年 (Water Tiger)4719 or 4659    — to —癸卯年 (Water Rabbit)4720 or 4660 Coptic calendar 1739–1740 Discordian calendar 3189 Ethiopian calendar 2015–2016 Hebrew calendar 5783–5784 Hindu calendars - Vikram Samvat 2079–2080 - Shaka Samvat 1944–1945 - Kali Yuga 5123–5124 Holocene calendar 12023 Igbo calendar 1023–1024 Iranian calendar 1401–1402 Islamic calendar 1444–1445 Japanese calendar Reiwa 5(令和５年) Javanese calendar 1956–1957 Juche calendar 112 Julian calendar Gregorian minus 13 days Korean calendar 4356 Minguo calendar ROC 112民國112年 Nanakshahi calendar 555 Thai solar calendar 2566 Tibetan calendar 阳水虎年(male Water-Tiger)2149 or 1768 or 996    — to —阴水兔年(female Water-Rabbit)2150 or 1769 or 997 Unix time 1672531200 – 1704067199

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

## Year length; leap years

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

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

### From Roman calendar

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

1. Richards 2013, p. 595.
2. Michael Douma (2008). Sally Smith (ed.). "The Christian Calendar | Calendars". WebExhibitsċċđ. 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 ${\displaystyle {\tfrac {1}{4}}}$  by about ${\displaystyle {\tfrac {1}{300}}}$ th of the sum of one day and night".