In Heinlein’s story “The Man Who Sold the Moon,” one ploy Harriman tries is to scare various people into thinking that a moon rocket could be used to paint the moon with coal dust (or something else dark) with various symbols (the hammer and sickle or the logo of a popular soft drink). This is purely a scare tactic, but today it occurred to me that the Moon is pretty dark already (albedo = 0.12 compared with about 0.5 for most of the other naked-eye objects in the solar system). Would it work better to paint parts of the Moon’s surface with glitter (or anything more reflective than the moon’s surface? You’d have bright against slightly less bright when the Moon is full - but you’d have visible features against a pitch black background during the new moon (from reflected earthlight), which would be quite dramatic (vice Harriman’s original scheme which would be ineffective during the new moon or close to it). Thoughts?
The perceived brightness of an object doesn’t scale linearly with the amount of light coming from it; it scales roughly logarithmically. In other words, for every “notch” you want to increase (or decrease) the brightness of an object, you have to multiply (or divide) the intensity of the light it emits.
In practical (?) terms, this means that the perceived increase in brightness from painting the moon with a perfectly reflective substance (albedo = 1) would be about the same as the perceived decrease in brightness by painting it with an albedo of about 0.014. (Since 0.014/0.12 ≈ 0.12/1.) So it’s not impossible to make black paint on the moon as high-contrast as white paint. The real question is which one your local hardware store will give you a better deal on.
The Earthshine idea is an interesting one. The above logarithmic scaling implies that the “light” and “dark” paint options would have equally high contrast against their background during the full moon and the new moon. However, the additional brightness of white paint during the new moon might make it more visible, since Earthshine is already rather dim and can get lost due to light pollution & atmospheric effects.
I’ve read that to see even a structure on the moon from the earth, it would have to be 100 kilometers in diameter. So, for a sign containing individual characters to be clearly visible on the moon, it would have to be gigantic, probably thousands of kilometers wide.
The plan was for the “sign” to be the size of the entire Moon, and for it to be a single, simple symbol.
Thanks @MikeS. Very illuminating (heh!).
I think glitter would be a terrible idea – it would act like lots of tiny mirrors, pointing in random directions. A lot of the light would end up scattered away from the line of sight of your prospective viewers.
Better would be to simply cover the surface with highly diffuse reflective material. White sand might work well, although arguably powdered Teflon (poly-tetrafluoroethylene, not any of the other substances that have been called “Teflon”) would be ideal – it’s an almost perfect Lambertian diffuse reflector across a huge wavelength range.
Actually, if your plan is to retroreflect Earthshine, rather than reflecting the much brighter sunlight, you might want to put a retroreflection system in place. We have left retroreflectors on the moon, and they work quite well. But corner cubes (the kind used) and “cat’s eye” retroreflectors would be too complex and expensive in bulk. The dirt-cheap way to do it would be to place down a layer of glass microbeads. Especially if placed atop a highly diffuse reflective layer (like the powdered Teflon), this would be a cheap, effective retroreflector, functioning much like the glass-bead paint they used to widely use for traffic crossings and the like.
It would still be grotesquely expensive to lift all that stuff to the moon and spread it out, even if you could manufacture much of it there (which I have my doubts about). And you’d only be reflecting earthshine, not the much brighter sunshine. But you might be able to make a “sign” light up during the new moon phase.
Wouldn’t “a bunch of mirrors aimed in all different directions” be effectively the same thing as “a highly diffuse reflective material”? At least, until you start putting retroreflector spheres on top of it.
Couldn’t we just sprinkle mold spores in the correct pattern and let the cheese turn blue there? It might take awhile for it to fill in the letters.
Finally a practical solution!
Depends upon how they’re pointed. A Lambertian diffuser is going to have its biggest component pointing straight up normal to the surface. I don’t know how the jumble of mirrors will be arranged.