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Tranquility Calendar

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The Tranquility Calendar is a solar calendar proposal for calendar reform proposed by Jeff Siggins in 1989.[1][2] It is a derivative of the International Fixed Calendar as well as the earlier Positivist Calendar published in 1849 by French philosopher Auguste Comte (1798–1857), providing for a year of 13 months of 28 days each, with one day at the end of each year belonging to no month or week, and a leap day approximately every 4 years.


The calendar's center date in history is called Moon Landing Day. The actual center point in time is the exact moment the word tranquility is mentioned in this somewhat famous quote — "Houston, Tranquility Base Here. The Eagle Has Landed". Moon Landing Day has 20 hours, 18 minutes, and 1.2 seconds Before Tranquility and 3 hours, 41 minutes, 58.8 seconds After Tranquility.

The calendar year has 13 months each with 28 days (divided in exactly 4 weeks) plus an extra day at the end of the year not belonging to any month (and so 365 days). The months are named after famous scientists in alphabetical order: Archimedes, Brahe, Copernicus, Darwin, Einstein, Faraday, Galileo, Hippocrates, Imhotep, Jung, Kepler, Lavoisier, and Mendel. The 365th day is called Armstrong Day; the leap day is called Aldrin Day.

Days that do not belong to a month are deemed to be outside the week. The first day of each year, Archimedes 1, is deemed a Friday and every subsequent day that belongs to a month is deemed to be in the conventional 7-day week.

The year starting the day after Moon Landing Day, and lasting until Armstrong Day, is designated 1 After Tranquility, or 1 AT, with subsequent years numbered in order; the year lasting from July 2016 until July 2017 is 48 AT, and the year lasting from July 2017 until July 2018 is 49 AT. The year ending the day before Moon Landing Day, and starting on the previous Armstrong Day, is 1 Before Tranquility, or 1 BT.

While Moon Landing Day's definition roughly correlates the calendar to Greenwich Mean Time, as there are no leap seconds, it more closely approximates TAI than UTC.

The leap year system matches the Gregorian Calendar; thus, the 400-year cycle of leap years starts on 31 AT - every four years is a leap year, except for every hundredth year which is not, except for every four hundredth year which is.

Because each month consists of exactly four weeks, the first day of each month and every seventh day after that for the rest of the month is deemed to be a Friday, the second day of each month and every seventh day after that for the rest of the month is deemed to be a Saturday, and so on. Therefore, each month begins on a Friday and ends on a Thursday.

This causes all months to look like this:

Fri Sat Sun Mon Tue Wed Thu
1 2 3 4 5 6 7
8 9 10 11 12 13 14
15 16 17 18 19 20 21
22 23 24 25 26 27 28

The 13 months and extra days occur on the following Gregorian dates:

Month Starts Ends
Archimedes July 21 August 17
Brahe August 18 September 14
Copernicus September 15 October 12
Darwin October 13 November 9
Einstein November 10 December 7
Faraday December 8 January 4
Galileo January 5 February 1
Hippocrates February 2 March 1
Aldrin Day February 29
Imhotep March 2 March 29
Jung March 30 April 26
Kepler April 27 May 24
Lavoisier May 25 June 21
Mendel June 22 July 19
Armstrong Day July 20


The several advantages of the Tranquility Calendar are mainly related to its organization.

  • The subdivision of the year is very regular and systematic:
    • Every year has exactly 52 weeks divided in 13 months.
    • Each month has exactly 28 days divided in 4 weeks.
    • Every day of the month falls on the same weekday in each month (i.e. the 17th always falls on a Tuesday).
  • The calendar is the same every year (perennial), unlike the annual Gregorian calendar, which differs from year to year. Hence, scheduling is easier for institutions and industries with extended production cycles.
  • Movable holidays celebrated on the nth certain weekday of a month, such as U.S. Thanksgiving day, would be able to have a fixed date while keeping their traditional weekday.
  • Statistical comparisons by months are more accurate, since all months contain exactly the same number of business days and weekends, likewise for comparisons by 13-week quarters.
  • Supporters of the Tranquility Calendar have argued that thirteen equal divisions of the year are superior to twelve unequal divisions in terms of monthly cash flow in the economy.


  • While each quarter would be equal in length (13 weeks), thirteen is a prime number, placing all activities currently done on a quarterly basis out of alignment with the months.
  • Christian, Islamic and Jewish leaders have historically opposed to the calendar, as their tradition of worshiping every seventh day would result in either the day of the week of worship changing from year to year, or eight days passing when Armstrong Day or Aldrin Day occurs.[3]
  • Birthdays, significant anniversaries, and other holidays would need to be recalculated as a result of a calendar reform, and would always be on the same day of the week. This could be problematic for Public holidays that would fall under non-working days under the new system; eg. If a public holiday is celebrated on January 8, under the Tranquility Calendar that holiday would always fall on a Sunday, Archimedes 8, which is already a non-working day, and compensatory leave would have to be given each year on Archimedes 9, thus essentially changing the date of the holiday to Archimedes 9.
  • A vast amount of administrative data, and the software that manages that, would have to be corrected/adjusted for the new system, potentially having to support both the IFC and the standard local time keeping systems for a period of time.


The collaborative science fiction writing project, Orion's Arm uses the Tranquility Calendar.[4]

See also[edit]

  • International Fixed Calendar
  • Leap week calendar
  • World Calendar


  1. Siggins, Jeff (July 1989). "Lunar Timekeeper: A Special Lunar Calendar for the Space Age". OMNI Magazine. 11 (10): 96. Archived from the original on March 9, 2015. Retrieved March 9, 2015.
  2. "OMNI cover July 1989 and table of contents". OMNI. July 1989. Archived from the original on May 10, 2012. Retrieved March 9, 2015.
  3. Joseph Hertz, Calendar Reform
  4. "The Tranquility Calendar". Orion's Arm. Archived from the original on March 6, 2015. Retrieved March 6, 2015.

External link[edit]

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