Extraplanetary calendars: “12 months per year” is an Earth-specific concept that will need adjustment in the space-age future!


By the year 2000, visionary futurists have estimated that over half the human population will live off-world, on another planet or in an orbital space colony.

The Issue:

Unfortunately, other planets generally have inconveniently not-matching-Earth orbital periods and day lengths. Thus, the calendar months will need to be adjusted in order for our intrepid miners on Mars know when to celebrate The Fourth of July or Cinco de Mayo (and possibly other month-and-day-specific holidays).


Various planets will require various adjustments to their calendars.

Let’s look at a few examples.

  • Earth: this is the most popular planet for humans. Day length: 1 day. Year length: 1 year. The year is divided into 12 months of ~30 days (Figure 1).
Fig. 1: Earth has a bunch of messed-up month names in English, like “October” (“The Eighth Month”) for the 10th month, but we’ve learned to deal with it.
  • Mercury: this is a weird one—the day length is longer than the orbital period. That means that the calendar only needs one “month” with a single day on it! Very economical. Downsides: your “word a day” calendar will actually only have one entry on it, so knowledge of esoteric vocabulary on this planet may be extremely limited.
  • Venus: this is another surprising one—apparently it spins around in 243 Earth days, but orbits the sun in 224 Earth days, so once again, we only need a single one-day month. Fortunately, we can just re-use the Mercury calendar here—this should save on logistics, since the Mercury / Venus calendar can be printed in a single batch (Figure 2) before being shipped by rocket to both planets.
Fig. 2: The entire calendar is just a single day. Very convenient!
  • Mars: this is the first situation where we’ll have to add months. A very reasonable 24 months (of 30-Mars-days-per-month) cover the whole orbit, so we’ll only need 12 new month names (Figure 3).
Fig. 3: We’ll need to come up with some new month names for Mars. Historically, some months were renamed to honor political figures—e.g. Julius Caesar (“July”) and Augustus Casear (“August”)—so perhaps this tradition will be continued by future Mars colonists who will, perhaps, name their months after Arnold Schwarzenegger (e.g. “Schwarzeneggtober”) and Clint Eastwood.

Unfortunately, the more distant planets have more inconvenient calendar requirements. Let’s look at Neptune as a representative outer planet:

  • Neptune: with 16 Earth hours per day and 165 Earth years per orbit, we’ll end up with 86,999 Neptune days per Neptune year. Thus, we’ll need 2900 months (86,999 / 30), as shown in Figure 4.
Fig. 4: Neptune’s calendar, with 2900 months, will (at one page per month) be more than twice as thick as the original publication of War and Peace. Citizens of Neptune will be unlikely to have much affinity for Fourth of July fireworks, since this date will only occur at most once in a single century.


When moving to another planet, it’s important to consider the calendar situation. Finally, this has been addressed!

PROS: Practical consideration of planetary month names may jumpstart space exploration, leading to the idyllic rockets-and-robots future promised by 1960s pulp science fiction paperback book covers.

CONS: This dreadful calendar situation may discourage space exploration: planetary explorers will have to give up not only their friends and family, but also any hope of ever seeing more than one additional New Year’s, May Day, Cinco de Mayo, or Fourth of July.