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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¢u 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¢ur 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¢ur 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¢u 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¢u 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.) |
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