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> @Logan:
>
> I would like to keep it as scientific as possible.  My thought on the
> changing orbital years is the influence of the planet's moons.  The changing
> severity of the orbital tilt would be a regular pattern of precession as on
> Earth, but it would have a much shorter period.
> The orbital changes are problematic, as the tidal forces might be
> disruptive.  However, I think that this could be dealt with by invoking
> resonances or something of that nature.

Moons, in the usual sense, won't change the orbit, although they could
certainly influence the axial tilt. It might be easier to arrange
without moons, though; having a large moon adds a lot more angular
inertia that makes the system as a whole more difficult to disrupt,
and planet-moon interactions tend to dampen on the order of a few
hundred million years. Interactions with other planets are mainly what
produce long-term axial precession; in the case of Saturn's moon
Hyperion, interactions with other moons result in chaotic rotation,
but that's largely influenced by it's extremely irregular shape,
something you won't have on a habitable planet, unless it's rotating
*really* fast.
For orbital changes, you could have a co-orbital moon/planet, as is
the pattern with some of Saturn's moons. That produces cycles that are
multiple years long, though, and in the two-body case, they're very
predictable. With more than two bodies involved... it would quickly
become chaotic, but those sorts of situations tend to be resolved by
something getting ejected. It's probably possible to set up a
dynamically stable system that wibbles around a couple of chaotic
attractors, but I have no idea how plausible that is.

-l.