How fast would a gas giant, or very close moon, need to orbit to maintain distance?

I have noticed that there is a common theme in science fiction art to depict one or more very large planetary bodies in very close proximity to the surface of the other world being depicted. (you can see this in the header artwork on this site)

My question is, (and I realize I am talking about fiction"¦) how realistic is this given the pull of gravity? It would seem that a binary planet or large moon of the relative size typically depicted would have to be orbiting extraordinarily fast in order to avoid collapsing into each other. Maybe this works in a still painting or computer generated image, but I have seen it in motion pictures where a giant orb is just hanging there, seemingly motionless in real time. Shouldn't there be enough relative motion to be able to perceive movement?

Is there a way calculate how rapidly a moon sized object would need to move across the sky if it were close enough to earth to appear as large as some common Sci-Fi scenes? Just for the sake of discussion, what if our current moon was close enough to appear 10 times larger, (probably still much smaller than some depictions I have seen) how long might a single orbit take?

posted about 4 years ago

Michael Hall‭

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