If a tidal locked terrestrial sized moon with 0 eccentricity, 0 incline, and was orbiting .01 AU away from a gas giant with 12 Jupiter masses (which would have approximately the same radius as Jupiter), how much of the sky would said gas giant take up if you stood at the center of the inward facing side? If this orbit is too close to be stable and/or makes the surface extremely volcanic (like Io) for a long time, I would appreciate any suggested orbit sizes that will fit these conditions, and then an answer based upon those conditions.

  • $\begingroup$ That's not a planet, it's a very small star. Given what you've stated about the physical size of the primary, about the same as Jupiter does from Callisto. $\endgroup$
    – Ash
    Oct 17, 2017 at 17:43
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    $\begingroup$ Essentially you're just asking "How big would an object the size of Jupiter appear from .01 AU away?" $\endgroup$
    – sphennings
    Oct 17, 2017 at 17:43
  • $\begingroup$ .01 AU is probably a bit close. It would experience a tidal force a bit less than half of what Io gets. I’d suggest moving it a bit further out. .02 AU would be fine, since tidal force varies with the inverse cube of distance. $\endgroup$
    – Mike Scott
    Oct 17, 2017 at 17:46
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    $\begingroup$ Sorry about answer, I was completely wrong, double checked it is the same view you'd get from Callisto, it's 0.013 A.U. from Jupiter. $\endgroup$
    – Ash
    Oct 17, 2017 at 18:03
  • $\begingroup$ It's a good idea to wait at least 24 hours before accepting an answer so that people from every time zone have a chance to answer. $\endgroup$
    – sphennings
    Oct 17, 2017 at 20:30

1 Answer 1


A gas giant with 12 Jupiter masses and 1 Jupiter diameter is fairly realistic, there are known exoplanets just like that.

From 1.5 million km, it will look huge in the sky. Its angular size would be about 5.5 degrees - 11 times bigger than full Moon, and more than 2 times bigger than Earth when seen from the Moon. (Angular Size Calculator)

At this distance, the satellite will be well outside its Roche limit (about 125000 km) and should be stable. (Roche limit formula)

With tidal lock and 0 eccentricity there is no worry about volcanism. Io is volcanic because its orbit is not a perfect circle, and this is what causing its tidal heating.

  • $\begingroup$ I really couldn't have asked for a better answer, thank you very much! If you wouldn't mind me asking, how did you come up with the answer for the angular size (as well as Roche limit), so I may be able to do it myself if I happen to change the distance or size of anything? Was it WolframAlpha? $\endgroup$ Oct 17, 2017 at 19:16
  • $\begingroup$ Io is also heated by it's interactions with other moons, namely the resonances with Europa, and Ganymede. $\endgroup$
    – Ash
    Oct 18, 2017 at 9:52

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