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Calendar and timekeeping facts
extracted from David Ewing Duncan: The Calendar
London: Fourth Estate, 1996 (pp. v-viii, 18, 277-278)

General year measurements | Length variations in natural years | Ancient calendar years |
Varying duration of different calendar years | Gregorian calendar correction dates |
 Year 2000 equivalents according to various calendars | When’s Easter? |


Go to the top of this documentGeneral year measurements

Length of the (tropical) year in 2000 AD

365 days, 5:48:45 (365.2422 days)
Time that the year has slowed since AD 1
10 seconds

Average decrease in the year due to a gradual slowing of the earth's rotation

½ second per century

Lunar Month

29 days, 12:44:2.9 (29.5306 days)

Length of year in Atomic Time (replaced Earth Time as official world standard in 1972)

290,091,200,500,000,000
oscillations of cesium

 

Go to the top of this documentLength variations in natural years

Type of year

Mean Time Interval (year 2000) in days

Lunar
354.3672
Sidereal
365.2564
Tropical
365.242199
Between vernal equinoxes
365.24237
Between summer solistices
365.24162
Between autumnal equinoxes
365.24201
Between winter solistices
365.24274

 

Go to the top of this documentAncient calendar years

Type of year
Duration

Ancient Egyptian year

365 days
Early Chinese year 354 days (lunar year) + days added at intervals to keep the lunar calendar aligned with seasons
Early Greek year 354 days, with days added
Jewish year 354 days, with days added
Early Roman year 304 days, amended in 700 BC to 355 days

 

Go to the top of this documentVarying duration of different calendar years

Type of year
Date
Duration
Error
Hipparchus' year
130 BC
365d 5hr 55m +0:06:14
Julian calendar year
45 BC
365d 6hrs +0:11:14
Ptolemy's year
139 AD
365d 5:55:13 +0:06:27
Aryabhata's year
499 AD
365d 8:36:30 +2:47:44
al Battani's year
882 AD
365d 5:48:24 -0:00:22
Omar Khayyámm's year
1100 AD
365d 5:49:12 +0:00:26
Alfonsine Tables year
1252 AD
365d 5:49:16 +0:00:31
Copernicus' year
1543 AD
365d 5:49:29 +0:00:43
Gregorian calendar
(Pope Gregory XIII)
1582 AD
365d 5:48:20 -0:00:26

 

Go to the top of this documentGregorian calendar correction dates

Date
Correction
45 BC 'Year of Confusion' (445 days). 80 days added to realign old Roman calendar with the solar year (Julian calendar).
1582 10 days (5-14 October) eliminated by Pope Gregory's papal bull to realign his calendar with the solar year.
1584 Most Catholic countries have accepted the Gregorian calendar
1752 British and American colonies accept Gregorian calendar. British Parliament eliminates 11 days (3-13 September) to realign.
1775 All Protestant Germany has accepted Gregorian calendar.
1873 Japan accepts Gregorian calendar.
1917 Russia accepts Gregorian calendar.
1971 Most recent Eastern Orthodox Church decision to reject the Gregorian calendar and retain the Julian calendar.
2002 Gregorian calendar 2 hours behind solar year
4909 Gregorian calendar 1 day behind solar year

 

 Go to the top of this documentYear 2000 equivalents according to various calendars

Calendar type
Year 2000=
First Egyptian
6236
Jewish
5760
Current Maya great cycle
5119
Ab urbe condita
2753
Ancient Babylonian
2749
Buddhist
2544
From Christ's birth in 4 BC
1997
Coptic
1716
Moslem
1420
Persian
1378
French revolutionary
208
Chinese
Dragon

 


 

 

 

 

 

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