Is there some astronomical / physics reason why it would (or would not) be realistic to have more than one "larger-than-Earth-moon" object in an alien sky as backdrop to a movie or tv show?

In other words, the view of Earth from our moon is quite impressive (takes up around 6 degrees of sky compared to how much the moon takes up in Earth's sky (0.5 degrees).

Often, to emphasize "you're now on an ALIEN planet," the sci fi writers will place multiple (usually 3) moons/planets in the sky. And they are usually shown larger than what the Moon would look like from Earth (so, larger than 0.5 degrees.)

Would this be realistic in the real world? In other words, in our solar system, Mars and Venus are so far away from us that they appear tiny, as stars.

However, if the Earth was a moon of Jupiter, then presumably, Jupiter would look huge in our sky, and if we also retained our moon, the Moon would look "moon-sized" in our sky, thereby having at least two large objects in our sky.

Or would an object the size/density of Earth not be able to both be a moon of a gas giant, while simultaneously having a large moon such as Luna?


migrated from scifi.stackexchange.com Apr 16 '18 at 20:35



Big objects in night skies

Well, consider, for example, a moon of Jupiter. This moon would have Jupiter in the sky, along with whatever assorted moons were bopping along that night. This would fulfill your requirements for plausibility.

To quote Wikipedia's convenient article on extraterrestrial skies:

For an observer on Io, the closest large moon to the planet, Jupiter's apparent diameter would be about 20° (38 times the visible diameter of the Moon, covering 1% of Io's sky). An observer on Metis, the innermost moon, would see Jupiter's apparent diameter increased to 68° (130 times the visible diameter of the Moon, covering 18% of Metis's sky).

Another example would be that of the skies of Enceladus, a moon of Saturn. To quote wikipedia again:

Seen from Enceladus, Saturn would have a visible diameter of almost 30°, sixty times more than the Moon visible from Earth [...] An observer located on Enceladus could also observe Mimas (the biggest satellite located inside Enceladus's orbit) transit in front of Saturn every 72 hours, on average. Its apparent size would be at most 26 minutes of arc, about the same size as the Moon seen from Earth. Pallene and Methone would appear nearly star-like (maximum 30 seconds of arc). Tethys, visible from Enceladus's anti-Saturnian side, would reach a maximum apparent size of about 64 minutes of arc, about twice that of the Moon as seen from the Earth.

Now, let's compare sizing of these moons and the Earth. Io has a radius equivalent to .286 Earth radii, and Enceladus a radius equivalent to 0.0395 Earth radii. So much smaller than Earth.

Earth size

This may seem mildly discouraging, but in fact, it is quite possible for an Earth sized object to orbit Jupiter. According to this reddit thread:

There's no reason I can think of why Jupiter couldn't have an Earth-sized moon.

In principle, I don't see a reason why a Jupiter-sized planet couldn't have an Earth-sized moon. The only limitation I can think of that would prevent an Earth-sized moon to orbit around Jupiter is the Roche Limit. The Roche Limit describes the distance at which an object would be ripped apart from the gravitational tidal forces exerted on it by the larger object. It's essentially the same principle as how our moon exerts tides on the ocean, but on a much larger scale.

If we assume that the Earth is a sphere with constant density (it isn't, but let's do this for easy calculations), we can use the equation you can find on the Wikipedia page to calculate the limit. Plugging in for the mass of jupiter (1.9x1027 kg), the mass of the Earth (6.0x1024 kg), and the radius of Earth (6378 km), we get that the Roche limit for the Jupiter/Earth system is about 5500 55000 km away from Jupiter's center of mass.

Since Jupiter's radius (69900 km) is much larger than the Roche limit, then there should be no problem with an Earth-sized satellite orbiting Jupiter at any arbitrary distance, as long as it wasn't actually touching Jupiter.

tl;dr: yes, it's quite reasonable for big objects to be in an earth-sized planet's alien sky.

  • $\begingroup$ So, it seems that a body could have at the most 'two large sky bodies' significantly bigger than our moon (0.5 degrees)? If Earth was orbiting Jupiter at a certain distance, Jupiter would certainly look bigger than 0.5, and if our moon was larger/closer than it is, that would qualify, but there really isn't anywhere to put a third body? (For example, the Enceladus - Mimas combination cited, the result is only 26 minutes of arc, smaller than our moon, and it would flit by once every 72 hours, not 'hang majestically in the sky' for the majority of a 60 minute network TV episode.) $\endgroup$ – Jerome Wiley Segovia May 7 '18 at 22:09

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