POSTSCRIPT: THE ULTIMATE CALENDAR
Beyond Mars

Designing a calendar for Mars turns out to be surprisingly difficult.

That is worth dwelling on.

Mars is, by Solar System standards, one of the easier cases. Its sol is reassuringly close to an Earth day. It has a recognisable year, seasons, axial tilt and a surface on which a permanent human civilisation could plausibly develop. Much of the language of terrestrial time remains useful.

And yet even here the problem refuses to collapse into one obviously correct answer.

Twelve months or twenty-four? Continuous weeks or recurring dates? Equal elapsed periods or stronger seasonal meaning? Familiarity or deliberate difference? The astronomy constrains the choices without resolving them.

Marian is one answer to those choices. Darian, Šurán, Levitt and others demonstrate that different answers can remain entirely coherent.

So what would happen if the problem became larger?

Not merely a calendar for Mars, but one capable of making sense across the Solar System.

Mercury, Venus, asteroids, the moons of the giant planets, distant dwarf planets and artificial habitats offer radically different combinations of rotation, orbit, illumination and season. Some natural cycles would be inconveniently short. Others would extend far beyond useful human planning horizons. Some places might not possess anything resembling the familiar relationship among day, month and year at all.

If producing one satisfactory civil calendar for a relatively Earth-like planet is this complicated, designing one calendar for every world may simply be impossible.

Perhaps that is the wrong question.

Perhaps the ultimate calendar would not require every world to use identical units or pretend that radically different astronomical environments are the same. Perhaps the deeper challenge is to discover which parts of civil time should remain local, which human rhythms ought to remain recognisable, and what must be shared so that different worlds can still understand one another.

There may be no universal calendar hidden in the Solar System, just as there is no uniquely correct calendar hidden in the orbit of Mars.

But there may be a more general architecture waiting to be found.

I have some ideas about how that might be done.

But that is for another time.