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Many science fiction movies involve scenes set on the surface of alien planets. In many of these scenes we see objects in the sky of the alien planet; sometimes a sun or suns; more often one or more alien moons. These objects are almost always very large compared to any standard of realistic physics.

The earth has a very large moon by known standards. It occupies an angle of about 0.5 degrees in the sky. It is, relative to the earth, the biggest moon in the solar system (and the biggest satellite we know). What we see in the sky on earth is therefore very large for a satellite.

But there are SciFi movies where we see alien skyscpaes with huge moons relative to the visual size of Earth's moon (which is very large).

And this is hardly a new thing. This is from Star Trek TOS:

Star Trek TOS image

This is from Total Recall:

Total Recall 1990 image (note that in this case we know how big the moon's of Mars are and it would be surprising if they can be seen at all from the surface: they are tiny).

This (the one that finally prompted me to ask this question) is from Altered Carbon and is a scene from Harlan's World:

Harlans World skyscape

Is there any good reason why SciFi moons are so big? Or is this a case of "I don't care about the laws of physics as long as it looks good on screen"?

Is there any in-universe justification for this common trope?

  • You're looking for an in-universe justification? Of what universe actually? – Napoleon Wilson Jun 8 '18 at 8:56
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    @matt_black the scene from TOS is actually from TOS-R, replacing a scene from TOS "The Cage" and "Requiem for Methuselah" that also showed a very large moon. Deimos has about 1/19th the apparent diameter of Earth's moon and appears star like from Mars's surface, while Phobos has about a third the apparent diameter of Earth's moon. They are clearly visible from Mars's surface. – M. A. Golding Jun 8 '18 at 19:47
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    I think that on the average most habitable planets will have either no moons or moons that look much smaller than Earth's moon. But some will have moons large and/or close enough to look much larger than Earth's moon. No doubt movie makers always choose those rare planets to set their films on. And filming through a telephoto lens can make Earth's moon or alien moons seem much larger than they are. – M. A. Golding Jun 9 '18 at 17:22
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    The pictures you've shown have them relatively close to the horizon. It's a well known optical illusion that moons look larger there. – nitind Jun 12 '18 at 4:08
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    @jeffronicus So you say. But that is a whole different question. ;-) – matt_black Jun 30 '18 at 10:34
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This is both an aesthetic choice, and a narrative strategy, utilized to emphasize the "alien-ness" of these speculative worlds. Similar strategy to utilizing multiple moons.

For my money, the original Starwars is still the exemplar (pre-digital, y'all):

double sunset on Tatooine

Here it's not the size of the celestial bodies, but the dual objects--Lucas is able to convey an absolutely convincing reality of the setting with physical props and real location, but the two suns that tell us this ain't earth.

  • It's a form of visual shorthand to reinforce the alien-ness of these locations

also:

  • Large moons are aesthetically appealing

ET and that kid flying in front of the moon

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    Those are the two suns of Tatooine. Not moons. – WakeDemons3 Jun 28 '18 at 20:55
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    @WakeDemons3 I acknowledge that they are suns, but the answer to the OP's question is not literal, and in no way confined to large moons. (i.e. we're talking about the symbolism, aka visual cues, used to convey narrative qualities of an alien location.) – DukeZhou Jun 28 '18 at 21:04

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