Julian calendar

calendar introduced by Julius Caesar in 45 BC
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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.

2024 in other calendars
Gregorian calendar2024
MMXXIV
Ab urbe condita2777
Armenian calendar1473
ԹՎ ՌՆՀԳ
Assyrian calendar6774
Bahá'í calendar180–181
Balinese saka calendar1945–1946
Bengali calendar1431
Berber calendar2974
British Regnal yearCha. 3 – 3 Cha. 3
Buddhist calendar2568
Burmese calendar1386
Byzantine calendar7532–7533
Chinese calendar癸卯(Water Rabbit)
4720 or 4660
    — to —
甲辰年 (Wood Dragon)
4721 or 4661
Coptic calendar1740–1741
Discordian calendar3190
Ethiopian calendar2016–2017
Hebrew calendar5784–5785
Hindu calendars
 - Vikram Samvat2080–2081
 - Shaka Samvat1945–1946
 - Kali Yuga5124–5125
Holocene calendar12024
Igbo calendar1024–1025
Iranian calendar1402–1403
Islamic calendar1445–1446
Japanese calendarReiwa 6
(令和6年)
Javanese calendar1957–1958
Juche calendar113
Julian calendarGregorian minus 13 days
Korean calendar4357
Minguo calendarROC 113
民國113年
Nanakshahi calendar556
Thai solar calendar2567
Tibetan calendar阴水兔年
(female Water-Rabbit)
2150 or 1769 or 997
    — to —
阳木龙年
(male Wood-Dragon)
2151 or 1770 or 998
Unix time1704067200 – 1735689599

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

Year length; leap years

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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 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

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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

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From Roman calendar

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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

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  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   by about  th of the sum of one day and night".