How large would a gas giant realistically appear to be on an Earth-like moon? I think a movie or two has portrayed them as very large, but that's an exaggeration, right? Depending on the size and orbital distance, is it possible that the gas giant might be barely visible during the day?

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    $\begingroup$ Google found this beauty as the first item in my search: spacemath.gsfc.nasa.gov/weekly/10Page51.pdf $\endgroup$
    – pojo-guy
    Apr 9, 2018 at 17:59
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    $\begingroup$ This depends on the sizes of the gasgiant and its moon as well as the distances evolved. You will find a detailed answer with pictures here: worldbuilding.stackexchange.com/questions/11505/… Could thus be also a duplicate. At least you should provide more details in your question (size of moon, gas giant, distances involved, etc.). $\endgroup$
    – DerGreif
    Apr 9, 2018 at 18:07
  • $\begingroup$ Can be readily calculated. refer to any high school trigonometry text, and plug in the gas giant's diameter and distance. Considering that Saturn is readily visible from Earth at night, and Jupiter is (usually) the 3rd brightest object in the night sky, after the Moon and Venus, it's going to be visible. $\endgroup$
    – jamesqf
    Apr 9, 2018 at 19:39

2 Answers 2


First, on brightness. Our Moon has an albedo of around 0.1 and is bright in the day sky. Jupiter's albedo is closer to 0.5, so it would be proportionately that much brighter. It should be easily visible in an Earth-like atmosphere.

Let's look at our own Solar System for sizes. Jupiter has a diameter of ~150,000 km. The Galilean satellites orbit at radii of 420,000, 670000, 1080,000, 1880,000 km. (All rounded numbers.) So they are respectively about 3, 4, 6, and 12 Jupiter diameters away. That gives Jupiter an angular diameter of 20 degrees, 12 degrees, 9 degrees, and 4 degrees respectively. (Compare that to the Moon's 1/2 degree size as viewed from Earth.)

We can do the same calculation for Saturn. (All sizes in 1000 km.) Diameter: 120, Ring diameter: 350, Distance to Titan: 1200. So Titan is 10 Saturn diameters away and 3 ring diameters. Saturn would be about 6 degrees across and the rings (nearly edge on, of course, so hard to see) around 20 degrees.

So Jupiter ranges from big to really big, but never gets to fills-the-whole-sky big. Saturn is bigish.


It would definitely depend on the size of the gas giant, and the distance from the gas giant to the moon. If the moon is distance D away from the world, and the gas giant has an apparent radius of R, then someone standing on the moon would see the gas giant subtending an arc of 2*arctan(R/D).


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