As you may be aware, there is little point in marking time on Venus by its motion relative to the Sun. The orbit of Venus is nearly a perfect circle, and the planet has no axial tilt, so it has no seasons. Any point along the orbit is essentially indistinguishable from any other one, and return to it has no special meaning. Thus, there is no solar calendar on Venus.
Instead, from the very beginning of the human settlement, the universally accepted calendar on Venus has been a geo calendar, based on the motion of Venus relative to Earth. The period of 584 days between the closest approaches (the inferior conjunctions) is termed a vear. Five vears are almost equal to eight Earth years. Thus, the conjunctions occur at the five points of a pentagon inscribed into the orbit (although, slowly drifting over time). The Era Venusienne (E.V.) timekeeping began the same vear the first member of Homo Venusienne, a genetically engineered Homo subspecies adjusted for life on Venus, was born.
It takes 13/5 of a turn around the Sun for Venus to arrive at the next conjunction, and 8/5 of an Earth year for Earth to do the same. Hence, there was a debate about which calendar to adopt, a 13-month calendar or a 16-month calendar. In the 13-month calendar, one month is equal to the average amount of time it takes Venus to pass from one point of its pentagon to the next one along the orbit: approximately, 45 days. The thirteen months are named after the Mayan zodiac constellations: Balam, Coz, Batz, Xibkay, Kutz, Mo’an, Keh, Dzec, Zotz, Aak, Tzub, Kan, Pek. They have 45 days each, except for the first one (Balam), which has 44 days. Due to the difference between the 584-day calendar and the synodic period of Venus (583.92 days), one day must be subtracted every 12.5 vears. This was done as follows: one day was subtracted from the middle month (Keh) every 13th, 38th, 63rd vear, etc. (in periods of 25), and one day was subtracted from the last month (Pek) every 25th, 50th, 75th vear, etc. But this calendar didn’t gain wide usage beyond the ultra-radical (even by the Venusienne standards) wizard orders, because of a loose correspondence to the relative positions of Venus and Earth.
The 16-month calendar, which eventually won, is based on the average amount of time it takes Earth to travel from one point of its conjunctions pentagon to the next one. This way, the middle of the vear corresponds to the superior conjunction, when Earth faces Venus across the Sun. Each 73-day period is divided into two months. The intercalation is similar to the 13-day calendar: one day is subtracted from the 8th month every 13th, 38th, 63rd vear, etc. and one day is subtracted from the last month every 25th, 50th, 75th vear, etc. The first vear began 12 hours earlier than the true time, so that the deviation from the true time does not normally exceed 12 hours in either direction.
The odd-numbered months are named after the cardinal and ordinal directions in the Breton language, whereas the even-numbered months are named after the same in the Estonian language. These belong to a group of very rare languages in which the ordinal directions (northwest, northeast, southwest, southeast) are not compound words as they are in English (northeast = north-east, for example). The months named after the cardinal directions (north, west, east, south) have 37 days each; those named after the ordinal directions have 36 days each. Since Venus orbits the Sun faster than Earth does, Venus leaves Earth behind along the orbit. As a result, Earth moves in the clockwise direction relative to the Sun-Venus line, when looking southward from the north pole. Therefore, the month names go from north to northeast to east… etc. to northwest. New Vear occurs between the two months named “north.”
Thus, the sixteen months of the Era Venusienne calendar are as follows:
- Põhi (“north” in Estonian): 37 days
- Vizh (“northeast” in Breton): 36 days
- Kirre (“northeast” in Estonian): 36 days
- Reter (“east” in Breton): 37 days
- Ida (“east” in Estonian): 37 days
- C’hevred (“southeast” in Breton): 36 days
- Kagu (“southeast” in Estonian): 36 days
- Su (“south” in Breton): 37 days (36 during some lapse vears)
- Lõuna (“south” in Estonian): 37 days
- Vervent (“southwest” in Breton): 36 days
- Edel (“southwest” in Estonian): 36 days
- Gornôg (“west” in Breton): 37 days
- Lääs (“west” in Estonian): 37 days
- Walarn (“northwest” in Breton): 36 days
- Loe (“northwest” in Estonian): 36 days
- Sterenn (“north” in Breton): 37 days (36 during some lapse vears)
Since Venus rotates very slowly around its axis (about once every 117 Earth days), the days are counted in 24-hour intervals, according to the biological rhythm of humans. Since the wake and sleep periods are regulated by artificial lighting, there is only one time zone on Venus, which is quite convenient for timekeeping.
Note, however, that due to the intercalation drift of 0.08 day per vear, the New Vear event doesn’t occur exactly at the same time as the corresponding inferior conjunction. For example, the 196/197 E.V. New Vear occurred at about 00:25 GMT on June 17, 2976 C.E., whereas the inferior conjunction occurred a little earlier, at about 20:06 GMT on June 16, 2976 C.E.