"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).

Suffice it to say, it would be quite a sight, especially
as the weather, rotation, and daily phase changes of the
gas giant would be very easy to see (barring dense local
cloud cover, of course). For us Earth folks such a sight
might almost seem oppressive, as it would take up a huge
portion of the sky and never go away, even at night,
except when it's cloudy (even then though the gas giant
would be so bright you'd probably still be able to see
it somewhat, like the Sun behind clouds).

For example, here's what Jupiter looks like from Io:
http://space.jpl.nasa.gov/cgi-bin/ws...8&day=23&centu
ry=20&decade=0&year=3&hour=07&minute=0&fovmul=1&rf ov=120&bfov=30&brite=1
- http://tinyurl.com/kzwu

Keep in mind that the FOV on that image is 120 deg, or
nearly the entire sky from horizon to horizon.

Here's what Earth's Moon looks like with the same
parameters, for comparison (centered in the image):
http://space.jpl.nasa.gov/cgi-bin/ws...8&day=9&centur
y=20&decade=0&year=3&hour=07&minute=0&fovmul=1&rfo v=120&bfov=30&brite=1
- http://tinyurl.com/kzwd


Though perhaps these are more obviously revealing at a
diferent scale:

Moon:
http://space.jpl.nasa.gov/cgi-bin/ws...8&day=9&centur
y=20&decade=0&year=3&hour=07&minute=0&fovmul=1&rfo v=30&bfov=30&brite=1
- http://tinyurl.com/kzwf

Saturn from Titan:
http://space.jpl.nasa.gov/cgi-bin/ws...8&day=23&centu
ry=20&decade=0&year=3&hour=07&minute=0&fovmul=1&rf ov=30&bfov=30&brite=1
- http://tinyurl.com/kzwi

Jupiter from Io:
http://space.jpl.nasa.gov/cgi-bin/ws...8&day=23&centu
ry=20&decade=0&year=3&hour=07&minute=0&fovmul=1&rf ov=30&bfov=30&brite=1
- http://tinyurl.com/kzwl


(P.S. Mind the wrap on those urls, or use the tinurl versions.)