On Feb 13, 12:21 pm, Crown-Horned Snorkack <chornedsnork...@[EMAIL PROTECTED]
>
wrote:
> What is the unit of rho, for the matter?
[kg/m^3], as I think I specified in the original post.
> Inside the Solar System, T is practically constant. P_rotational is
> unknown and unknowable.
Well, perhaps not. You could make the argument that T is constant at
about 4.6 Ga... but that would certainly seem untrue for Triton, for
instance. P_rotational may not be exactly specified, but there is a
well known empirical relation****p that seems to hold for tidally
unbraked systems (Jupiter, Saturn, Mars). I'll give you it's sketchy
at best, but for worldbuilding you have to start somewhere, and I
usually like starting at an assumed state and seeing what happens
forward.
> Venus, with 228 days orbit, has free rotation - even though slow free
> rotation should be easily stopped by even weak tides.
Well... again, not exactly. The last stages of tidal evolution are the
slowest, because the offset between the tidal bulges and the primary-
secondary line is very small. For Venus, I'm not sure what the answer
is - some maintain it has to due with coupling to thermal tides in the
atmosphere, some to frozen in tides coupling weakly to the Earth, and
some assume it's just a last-body-impactor issue.
> Mercury, with 88 days orbit, is locked - but locked to 3:2, not 1:1.
Umm... true. But locked is locked, from that standpoint.
> Moon, at 27 days, is locked 1:1. So is Iapetus, at 79 days.
> However, Hyperion, with just 21 days orbit, rotates freely.
Well, again... no. Hyperion is in a 3:4 orbital resonance which seems
to lock it into a chaotic, not free (or even constant) rotation.
The equation I quoted is an approximation from a text that assumes a
very simple 2-body tidal model. The fact that the real world is a heck
of a lot more complex doesn't rule it out, it just means that, as
always, you need to be careful about it and watch your assumptions.
--
Brian Davis
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