Re: Programming a calendar system

On Wed, Apr 28, 2004 at 02:06:51PM +0200, Carsten Becker wrote:
> SQRT of the 3rd power of 1.45 etc. looks very much
> like one of Kepler's laws ... Not caring about any astronomical law I
> set 6 days = 1 week, averagely 25,333... days = 1 month, 18 months = 1
> year and 1 year = 456,25 days. Every 4th year one day is dropped.
That particular Keplerian law tells you how long a planet's orbital
period ("year") is, based on its distance from the sun.  If you express
the period (commonly represented in equations by T for Time) in Earth
years and the distance (commonly represented by R for Radius) in
Astronomical Units, the constants cancel out and you are left with the
very simple relation that T^2 = R^3.  You can, of course, go the other
way as well.   In your case, you have a 455.75-"day" year, where each
"day" is 27 "hours" of 18 "minutes" of 72 "seconds" of 1.2 SI seconds
each.  That's a total of 19,137,124.8 SI seconds, which is about 221.5
Earth days, which is about 0.6064 Earth year.  So the planet of the
Aregans must be 0.6064^2^(1/3) = 0.5325 AU from their Sun.  That means
that their Sun has to be a K-type (red) star.   Which means that their
Sun is much bigger than ours as well as much closer to them, so would
be much more dominant in the daytime sky.

But there's no astronomical law governing the length of a rotation.
Well, that's not quite true; there is a particular orbital distance
called the "tidal lock radius".  Planets orbiting at that distance
eventually become locked to the Sun like the Earth is locked to the
moon; the same side of the planet is always facing the sun.  Which means
that 1 day = 1 year.  For planets inside the Tidal lock radius,
the day is actually longer than the year; this is true of Mercury
and Venus, for instance.  Beyond that, however, there's no reason you
can't make your days any length you want.  And "weeks" and "months"
are purely artificial constructs that can be anything you want.
Sure, on Earth the size of the week was inspired by the number of
visible planets in the sky, and the size of the month by the phases of
our moon, but there's no reason to think those things would influence the
calendar of another species on a different planet.

I view astronomy as a source for inspiration rather than a constraint.
For instance, I decided a long time ago that my conplanet Dankar would
orbit the star mu Tauri - that is, the 12th-brightest (mu is the 12th
letter of the Greek alphabet) star in the constellation of Taurus.  How
did I make this decision?  Well, mu is my first initial in Greek, and my
astrological Sun sign is Taurus.  :)  I mainly wanted to avoid all the
really bright "famous" stars, especially the ones with proper names
(like Aldebaran, a.k.a. alpha Tauri), because I wanted a system that
hadn't been used before in an SF context.

For a long time I had no actual information about my chosen star;
before the Internet it was harder to find such info about
anything but the brighter stars, even at the local library.
But I did eventually discover that mu Tauri is a B3 star.
Whups!  Problem!  B-class stars only live about 10 million years or so;
even if planets manage to form in that time there's nowhere near enough
time for life to evolve.  But instead of being deterred, I decided to
make this impossibility part of the story: there's a planet orbiting
mu Tauri, and it has humanoid (in fact, Human) life!  How did it happen?
The planet must be artificial - who created it?  The inhabitants are
genetically Terran - were they brought there by the people who created
the planet?  When?  etc.

Incidentally, the life zone for a B-series star is pretty far out;
Dankar orbits at about 4 AUs, which means its year is about 8 Earth
years long.  But they use years much the way we use decades; they reckon
age and such in more manageable units equivalent to our "seasons",
except that the Dankarans divide their year up into six seasons
instead of four.

-Mark

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