What would an eclipse with a translucent crystal moon look like?

In one of my worlds, a special moon orbits the planet — it's made of a translucent crystal, blocking only 80% of the incoming light. It's purple. And on certain days, just like earth, this moon covers the star the planet calls home, and creates an eclipse. What would this eclipse look like?

More info if you want:

  • This isn't a question about how the moon is made of crystal; that's handwaved for the purposes of this question.
  • At the center, the moon blocks 80% of the light, but towards the sides this number drops off as the crystal gets thinner.
  • It's a total solar eclipse.
  • Assume star = sun, people viewing = humans, and planet = earth if it matters.
  • If you need more info, ask.

2 Answers 2


The focal length of a ball lens of radius R with index of refraction n is nR/(2(n−1)), and if the planet/moon is "just like earth", then the distance from the moon to the earth is approximately 200*R. So there are three scenarios, depending on the properties of your magical crystal.

  1. n > ~1.005 : (most likely the case if the crystal is similar in properties to traditional transparent/lucent materials). In this case most of the light that isn't directly blocked by the moon will be refracted away from the earth, so the eclipse will look similar to that produced by an opaque moon.

  2. ~1.005 > n > ~1.001 : (the oops scanario). In this case a significant proportion of the un-blocked light will be focused onto the earth so the moon will appear as a bright object in some regions of the planets surface. The situation will get more 'interesting' as n approaches 1.0025. In particular, if n is very close to 1.0025 then there could be a small patch of the earth's surface where the intensity of refracted light could be significantly greater that that of the naked sun (images of ants under magnifying glass type of scenario).

  3. ~1.001 > n : (the more artistic scenario - needs a refractive index not dissimilar to that of the vacuum). In this case the moon will not appear to significantly refract the light, so during the eclipse it will appear as a big ball thar is brighter (whiter) towards the edges and darker (purpler) towards teh centre.


This will actually look remarkably like a total eclipse on Earth.

Since the crystal the moon is made out of presumably has a refractive index greater than one (quartz is 1.46, sapphire 1.77 and diamond 2.42) and the moon is approximately spherical, it will act as a lens.

We don't know the focal length of the lens, but any plausible value will be much less than the moon-to-planet distance, so the light passing through the moon will be widely dispersed and the moon will look dark. Just like an opaque moon.

And that's ignoring the issue of the crystal moon being totally opaque if it's actually made of any known kind of crystal. A thickness of more than a few tens of feet would absorb all the light.

If the moon were not solid, but a shell a few tens of feet thick, it would have a much longer focal length, dependent on the shell's thickness. Sadly, such a shell the size of Earth's moon will collapse under its own gravity, into a disorganised ball of fragments.

  • 2
    $\begingroup$ A piece of quartz, sapphire and diamond even ten meters across would be completely opaque anyway. $\endgroup$
    – AlexP
    Sep 5, 2022 at 23:15
  • 5
    $\begingroup$ @JBH: The focal length of a ball lens of radius $R$ with index of refraction $n$ is $\frac{nR}{2(n-1)}$. Of course we don't know the index of refraction of the ultrasupertransparentium the wondrous moon is made of; assuming something tiny like 1.01, the focal length is about $50R$, decreasing drastically to $1.5R$ if the material has a reasonable index of refraction of 1.5. $\endgroup$
    – AlexP
    Sep 5, 2022 at 23:31
  • 3
    $\begingroup$ To add to AlexP's comment: for scale, distance to Earth's moon is $210R$. $\endgroup$
    – tylisirn
    Sep 6, 2022 at 0:56
  • $\begingroup$ This Wiki article en.wikipedia.org/wiki/Underwater_vision#Visibility claims that 80 m is about the upper limit of visibility of an object through water. If the "crystal moon" had a density far below that of water it would be expected not to retain its spherical shape and get blown around by solar wind. $\endgroup$
    – BillOnne
    Sep 6, 2022 at 20:10
  • $\begingroup$ The glass used for optical fibers in the visible spectrum used to have a loss of about 3db per kilometer. It is some of the purest glass in the world. So you would lose about 1/2 the light each kilometer. For the longer wavelength used for optical communications it is much much better. For pretty much any other material as Alex P pointed out it you would be opaque pretty quick. But it is an artificial moon anyway maybe it doesn’t need to be solid. $\endgroup$
    – UVphoton
    Sep 6, 2022 at 21:37

You must log in to answer this question.

Not the answer you're looking for? Browse other questions tagged .