Christopher M. Jones wrote:
"Martha H Adams" wrote:

Every time I see an image like the giant moon on the cover of the
August 2003 issue of Locus, I think it makes a wonderful image but
then I ask, but can that really happen?

Which leads me to ask a couple of questions.

1) If a moon is orbiting near Roches' limit, what does it look like?
I guess, kind of odd, noticeably egg-shaped. Not nicely spherical
as on the Locus cover.

2) Is there a characteristic solid-angular size, a maximum limit on
how large an orbiting neighbor can appear, before it begins to
break up? I.e., how much sky it fills?

Seems to me, I recall an sf story which had two worlds orbiting
practically in contact, with an ocean spilling back and forth between
them. They shared an atmosphere. Robert Forward, I think.


Haha! Flip it around and come up with something different.
A habitable Earth-like world as a moon of a gas giant. If
you mess around with the numbers enough it's not too
unreasonable to get a moon around a gas-giant planet
(probably Saturn size or maybe a bit bigger) which is
tidally locked to it (as most such moons are) but has an
orbit / solar day close to one Earth day (and, of course,
the whole thing is appropriately distant from the parent
star to give an Earth-like climate). For people living on
the planet side of the Earth-like moon the gas giant would
hang in the sky much, much larger than our moon and would
remain in the same position permanently (with added buts
concerning libration and eccentric orbits and all that).

I believe the cartographers for tide locked earth like moons would make
interesting globes.

On earth 0 degrees longitude is arbitrary. But on a tide locked earth
like moon, the point on the equator closest to the primary could be a
non arbitrary choice. The point furthest from the primary would be 180
degrees. There would be three great circles: the equator, a great circle
containing the poles and points nearest and farthests from the primary,
and the third great circle cuts the moon in half along a plane
perpendicular to the primary's radius vector. (I think of this 3rd great
circle as a "terminator" also that usually means a plane perpendicular
to the sun vector)

The globe would look like an inflated octahedron.

For the moon's day night cycle would be of the same duration as its
period about the primary. Mimas has close to a 24 hour period about Saturn.

Hop
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