Is big moon in sky plausible?

"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.)

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.

Cheers -- Martha Adams

Good morning,

On Sat, 23 Aug 2003, 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?
[...]

For those (including yours truly) who do not read Locus regularly, here's
the image in question:
http://www.locusmag.com/2003/Issue08/cover511big.gif

The first thought that comes to mind is the photographic technique to
create a picture of a "big moon" behind a silhouetted house or grove or
what-have-you. Bearing in mind that the angular width of the moon is (for
all practical purposes) the same, regardless of your location, you travel
an appropriate distance from the house to be in the photo, such that the
house has a narrower angular width than the moon (and such that nothing
blocks your view of the house). Use a lens with a sufficiently large
magnifying power that the image will occupy most of the frame (or use a
film with a sufficiently small grain that you'll be able to blow up the
image in the darkroom to fill the photograph), and take the picture when
the moon is behind the house.

Of course, such a technique would also make the sun(?) in the Locus cover
image appear abnormally large.

Interesting conclusion we can draw from that cover image -- the star we
see must not be the primary source of light, or
1) the surface of the moon(?) would not be so visible (though this might
be dismissed as the planet having suffient albedo to reflect light onto
the moon), and
2) there wouldn't be specular reflection on the vehicles' hulls on the
viewer's side.


Take care,
cb

--
Christopher A. Bohn ____________|____________
http://www.cis.ohio-state.edu/~bohn/ ' ** ** " (o) " ** ** '
"To be prepared for war is one of the most effectual means of
preserving the peace." - George Washington, 1790

"Rui Pedro Mendes Salgueiro" wrote:
[snip habitable gas giant moon stuff]

This was used by Isaac Asimov in his book "Nemesis":

http://www.amazon.co.uk/exec/obidos/ASIN/0785796576/

It was also used by RAH in Farmer in the Sky, after a
fashion.

Christopher A. Bohn said:
The first thought that comes to mind is the photographic technique to
create a picture of a "big moon" behind a silhouetted house [...]
Use a lens with a sufficiently large
magnifying power that the image will occupy most of the frame [...]
and take the picture when the moon is behind the house.

An old photographer's rule of thumb: the image of the Moon on the
negative is 1 mm for each inch of the lens' focal length. For the
traditional 35mm camera with a 2" (~50mm) focal length lens, the Moon
is about 2mm wide.

A 35mm frame is 24mm wide, so you want something around 20 inches /
500mm to nearly fill the frame. Coincidentally, that's about the focal
length where a mirror lens (basically a refined Newtonian reflecting
telescope) becomes a reasonable alternative to traditional lenses.
--
Kevin Willoughby oSpam

Imagine that, a FROG ON-OFF switch, hardly the work
for test pilots. -- Mike Collins

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
http://clowder.net/hop/index.html

"Hop David" wrote:
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.

That's not the half of it. Think about the troubles of
navigation. For millenia it was difficult to determine
both longitude and latitude on the open ocean. Latitude
was the easiest to determine with crude methods (height
of the Sun above the horizon at noon, etc.) and
travelling along one partituclar line of latitude was
the easiest form of oceanic navigation for a long time.
But having a gigantic planet in the sky adds a new twist.
Just look up and you can see what time it is at 0
longitude, plus, you can just look at the position in
the sky of the planet and know what your position on the
moon is fairly easily. That would make early map making
and long distance travel a lot easier.

Christopher M. Jones wrote:

That's not the half of it. Think about the troubles of
navigation. For millenia it was difficult to determine
both longitude and latitude on the open ocean. Latitude
was the easiest to determine with crude methods (height
of the Sun above the horizon at noon, etc.) and
travelling along one partituclar line of latitude was
the easiest form of oceanic navigation for a long time.
But having a gigantic planet in the sky adds a new twist.
Just look up and you can see what time it is at 0
longitude, plus, you can just look at the position in
the sky of the planet and know what your position on the
moon is fairly easily. That would make early map making
and long distance travel a lot easier.




On the half of the planet facing the moon, yes...but the people on the
far side would still have navigation problems....on the upside, having
such an object in the sky at noting it's changing position as one
traveled would make the fact that you were living on a sphere (Okay...a
bit of an ellipsoidal sphere maybe) fairly obvious at an early phase of
civilization.
Pat