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I have an earth-like planet in a fantasy setting with two moons, one of them made of crystal. It will be a fantasy crystal, properties TBD (probably by some of the answers to this question).

My main question is, would I be "ruining" night time by putting a solid crystal moon in the sky of my world? I'm not trying to fully turn night into day here.

I'm not sure I can really picture what this would look like, how the sun would interact with a crystal moon. I imagine, no more moon phases... but would the intensity of light from the moon change depending on the angle the sun hits it (not looking for math just in general)? Could it cast prismatic rainbows on the planet sometimes or all the time? And would it be dangerous at a solar eclipse, whether from intensified sun beams or extreme brightness?

If this moon would be too bright, is there a compromise I could achieve, like say, making the moon just 50% crystal? Or making sure the core is fully opaque and can't transmit light through the entire moon?

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    $\begingroup$ You may be asking the Q a bit early. For example, how pure is the crystal? A perfectly pure crystal (no reflectivity, 100% transmission of light) wouldn't even be visible other than (maybe) refraction through the sphere. How hard is the crystal? Can it withstand meteor strikes? Are there craters? Chips? Cracks? I can see a lot of fun with this. Forget "realism," that depends to much on knowing the exact chemistry of the moon (which you haven't supplied). Focus instead on what fun you can have with crystal. Is it a smooth ball or does it have facets? It might be more like glitter in the sky. $\endgroup$
    – JBH
    Jan 23, 2022 at 21:14
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    $\begingroup$ Our very own Moon is made of crystalline materials. If by "crystal" you mean glass, then say so. $\endgroup$
    – AlexP
    Jan 23, 2022 at 22:39
  • $\begingroup$ The moon IS made of crystal. It's heavily silica, much as sand on Earth is. It's just that it has been blasted by metorites for billions of years, so the crystals are all very small and oddly shaped. Just like sand is. Also, with naturally occuring crystal, you won't be able to see much through more than a few km. It's dark at the bottom of the ocean through just a few km of water, even in the mid Pacific where the water is very clean. $\endgroup$
    – Dan
    Jan 24, 2022 at 5:05

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Refraction/diffraction

If you have a moon that consists of a single crystal, the sun may shine through as diffuse light during a solar eclipse. The crystal would show its color, like it does with the full moon, but there could be additional shades of different color, inside. To have this effect, the crystal should be high level of purity. A crystal moon polluted, or consisting of many crystals will not be translucent.

Reflection

Reflection will depend on type of gemstone the moon consists of. Color is most important, white will shine.. maybe too much. In any case, don't expect a shiny surface. An extremely hard stone like diamond could preserve its crystalline surface for some time, but in space, the issue is high energy particles. Diamonds cannot handle energetic protons. The shape will be preserved, though, resulting in square patterns shining through. Diamonds can have many colors, ruby's are red or blue. Inhabitants of the planet would enjoy their colored moon every month, shades seem to glow over its surface. The edge of craters are sharp, sometimes you see sparkles of light. It will be brighter than the moon, I guess.. Our moon reflects only about 3-12% of the sunlight. Planet Earth would reflect 20-26%, considerably more. Polished copper can do 70%, so you could expect a tenfold increase (or thereabout) of the moonlight, especially when the crystal is homogeneous and having a light color.

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  • $\begingroup$ "If you have a moon that consists of a single crystal, the sun may shine through as diffuse light during a solar eclipse": That depends on how large the object is. An object the size of our own Moon would be opaque even if it were made of the best optical glass that Zeiss can make. $\endgroup$
    – AlexP
    Jan 23, 2022 at 23:15
  • $\begingroup$ @AlexP we agree reflection will be more prominent.. depending on color.. but the sun, in case of a solar eclipse, is a powerful light source.. I wonder what would happen with a monocrystalline piece of diamond, or quartz.. $\endgroup$
    – Goodies
    Jan 23, 2022 at 23:19
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It depends on the shape and material of the crystal, the more pure the crystal and the more transparent it will be, making it semi-invisible in the sky acting like a giant magnifying glass, burning people like ants. More realistically, it wont be pure and won't be of the perfect shape, reflecting a lot of light but not enough to burn someone alive, maybe just enough to give them cancer through excess radiation. Even more realistically, it will be covered in dust from million of years of meteorite impacts distrupting the surface, making it into a normal moon. There are planets made of ''diamonds'' in the real universe, and they don't look like clear crystals.

enter image description here

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  • $\begingroup$ "The more pure the crystal and the more transparent it will be": it is very far from that simple. An object made of pure crystals of gold is not transparent at all. An object made of pure crystals of diamond might be transparent, if it is one not very large monocrystal, but will be opaque if it is either made of many little crystals or if it is one large monocrystal. (And in general, an object the size of the Moon will be opaque no matter what it is made of.) $\endgroup$
    – AlexP
    Jan 23, 2022 at 22:40
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Your Moon would look just like ours.

Light only penetrates transparent/translucent materials up to a depth, which varies by material. Water is quite transparent for a few meters but even on the clearest lakes you can't see much deeper into them. Our atmosphere is perfectly transparent for more than a hundred kilometers, but when it gets thick you start losing some colors. This is why sunsets are reddish.

Consider Jupiter. Its atmosphere is as transparent as ours. We can see a few hundred kilometers into it and then we just see red and brownish patterns.

Solid crystals tend to be less transparent than gases. Add to that, any steroids and comets impacting on that Moon will deposit non-transparent impurities on the lunar surface.

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Night will globally still exist

It just may be a little brigther over the whole globe or there will be thin burning line somewhere and dark night at the rest. Or anything between including rainbow effects and such.

But if it is the "moon" and not "co-planet", it means it is way smaller, than the earth - so it can get a way smaller angle of sun light (that means way less of sun energy) - an so even if it somehow reflect ALL of it to the earth, it still be way less, than Sun shine thru the day.

So "day" and "night" would still be vissibly different on the globe globally.

It may by possible (with some speacial cases) to have some area(s) on globe, which would be light at night as well as at day, if the moon would reflect large portion of its share of Sun light there, but it would be just about size of the moon (at 100% effectivity), with rest of globe in dark.

Or the night may be little lighter globally, but not as much as at day.

Anyway there is lot of stars at sky, so night is not totally dark anyway, it just may seem so to eyes, that are not sensitive enought. (And there is a problem with sensitivity range, if you want to see both under just stars and on full sunshine, so it is usually evolutionary impractical have such effective (and cost) eyes if you can cope some other way)

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Surface, refractive index, angle of incidence, transparency of the crystal, color of the crystal

The amount of light received by the planet depends on type of surface, refractive index of crystal, angle of incidence of light, transparency of the crystal, color of the crystal etc.

Rough surface

If the surface is rough and unpolished, then reflection will be diffused. You will see a brighter moon but not very much (eye's response is logarithmic).

Polished surface

If the surface is smooth, polished, spherical and the crystal is transparent (like quartz, diamond, ice, sapphire), the moon will act like a big double-sided convex lens. Most of the light will pass through and little will be reflected. Light may disperse at certain angles of incidence (like in water droplets after rain causing a rainbow) and you may see rainbow like colors.

Surface with large smooth planes

If the surface of the moon is in the form of large smooth planes, then specular reflection, refraction, total internal refraction and dispersion will occur depending on the refractive index of the crystal and angle of incidence. You will see a multicolored glowing ball.

Solar Eclipse

When solar eclipse occurs, the sun will not be black but dimmed because light incident at angles greater than critical angles will have total internal reflection.

Only light incident at angles less than critical angles will pass through.

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