System & Planet

Calendar of Joi, 61 Ursae Majoris


The planet Joi has a sidereal day (the time for the planet to rotate once about its axis) of 28 hours 48 minutes which equates to a solar day (the time from noon to noon) of 28 hours 55 minutes and 57 seconds. With a year of 262.254 Earth-days (217.545 local days) the standard Terran calendars were clearly inappropriate for use on Joi.

The three Joian moons (Blanche, Argent and Or) have rotational periods of 10.8, 16.3 and 83.8 Earth days respectively. These equate to lunar months (the time for the moon to reach the same point in the sky with the same phase) of 9.34, 14.41 and 102.02 local days respectively.

ESA Calendar

The basic form of the calendar was determined by the follow up ESA survey team who needed to define a simple way of marking time on the planet.

Years were counted from the date that the ESAS Argonaute (the original survey vessel) first entered orbit on the 3rd of October 2226. That planetary year was defined to be year 0 with the start of the year taken to be the Vernal Equinox (22nd of September 2226). The Vernal (Spring) Equinox was chosen as it conventionally represents the zero point in celestial co-ordinate systems.

Each year was divided into four seasons (spring, summer, autumn and winter) and each season into three months (simply numered one to twelve) each of eighteen days. In addition odd numbered years had one extra leap day while even numbered years had two extra leap days at the years's end. Additional leap days were added for years ending in 20, 40, 60, 80 or 200. The familiar twelve month cycle was retained due to its usefullness in referring to seasonal variations of weather and biological activity.

The day was divided into 29 hours, 28 of them a Terran Standard hour long and the 29th with only 56 minutes (actually 55 minutes 56.93 seconds). Noon was defined as 14:00 so that the 29th "hour" spanned 28 minutes either side of midnight (thus midnight would be 28:28). The 29th hour was colloquially known as the "midnight hour" or the "short hour".

This was as far as the ESA system officially got although the staff on the ground also tended to refer to the 12 months by the relevant names in their respective native languages.

Colonial Calendars

The five Joian colonies all took the ESA calendar as the basis for their own however they each adapted it to their national character. The ESA calendar is retained for international transactions and is writen in the European form day/month/year usually preceded by the ESA initials to avoid confusion with national dating systems. Leap days are assigned to month 13 thus in year 19 there would be only a single leap day - 1/13/19 - while in year 20 there would be three (one normal day, one for the even year and one for year 20) leap days 1/13/20, 2/13/20 and 3/13/20.

Elysia (Former French Colony)

The Elysian calendar was determined by a commitee set up by the French Colonial Office in 2239. The committee recommended that the calendar should be based upon that used by France in the period following the first French Revolution (originally proposed by Fabre d'Eglantine in 1793). The original had three ten day weeks (decades) per month and twelve months per year plus a five day (six in leap years) festival at the year's end. Unlike the ESA calendar the revolutionary year began at the Autumnal Equinox rather than the Vernal. Thus only relatively minor modifications were required to fit the ESA framework.

The Elysian Calendar retains the ESA's twelve months but splits each into two nine day weeks. The additional leap days are placed at the end of the year. The year begins at the Vernal Equinox and the year numbering follows that begun by the ESA (ie number of local years since discovery).

The names of the months (starting from the beginning of the year in the middle of spring) are those originally proposed in 1793.

Floreal (Flowering) - Spring

Prairial (Pasture) - Spring

Messidor (Harvest) - Summer

Thermidor (Heat) - Summer

Fructidor (Fruit) - Summer

Vendemiaire (Grape Harvest) - Autumn

Brumaire (Fog) - Autumn

Frimaire (Frost) - Autumn

Nivose (Snow) - Winter

Pluviose (Rain) - Winter

Ventose (Wind) - Winter

Germinal (Germination) - Spring

The additonal leap day or days at the end of the year are known as Les Jours de Decouvert (Discovery Days) and are assigned to Decouvert (Discovery) month.

The date is written , in the author's preferred combination of numerals and names, day/month/year and is numerically identical to the ESA system. Thus 19 Pluviose 78 would be the 19th day of Pluviose (the tenth month) in the 78th year since discovery.

Each normal week consists of nine days named arithmetically (primidi, duodi, tridi, quartidi, quintidi, sextidi, septidi, octidi, nonidi). Quartidi is set to be a half work day while octidi and nonidi are rest days (giving a similar rest to work ratio as the Terrestrial week and weekend).

The standard ESA 29 hour clock was retained with the normal working day set to 08:00 to 13:00 and 15:00 to 20:00 (14:00 being the local noon).

Design Notes

Firstly, and most importantly, my thanks to KevinC for pointing out that the original version of the calendar had neglected to use solar days and lunar months. He also devised the leap year system used here as my original used the wrong day length.


I have retained the twelve months as I think that they give a useful guide to weather and seasonal activity (reinforced by the poetic names of the months). Rather than retain the traditional month names I opted for the revoultionary names to reinforce the differences between Earth and Joi, also a year beginning on 1st April would be confusing (to anyone who wasn't an accountant at least). The month names are obviously only relevant to the southern hemisphere (where Elysia is situated) which includes the British and Azanian colonies. The northern hemisphere colonies (Germany and Japan) will obviously be seasonally opposite.


Given an 18 day month the obvious decision is whether to make the week six or nine days in length. I had orignally opted for nine days as the moon Blanche has a nine local day orbital period and so would serve as a useful marker. However as KevinC pointed out I had neglected to account for the rotation of the planet and this increases the lunar month (as seen from the ground) slightly to 9.34 local days. So one reason for the nine day week was inertia the other is that it more closely fits the Revolutionay ten day week.


I have retained Earth hours, minutes and seconds as they are now international standards based on physical measurements (specific number of vibrations of a particular atom at a particular temperature) rather than the rotation of a particular planet (Earth). You could, as KevinC suggested, use Joi hours, minutes and seconds which greatly simplifies navigational calculations. However a Joian time unit would by 1.20 times its Earthly equivalent which would have knock on effects on any other time related measurement (eg vehicle speeds, heart beat rates etc).

Version 1.0


Copyright J.M. Pearson, 2001

System & Planet