> About 1 atom per cm³ and enough photons to read by.
Do you have a citation (or better yet, a calculation) for this? I’ve been searching for about 10 minutes and and it doesn’t quite make sense.
The full moon typically casts about 0.05 - 0.1 lx.[1] With perseverence and some eye strain, I think I could read by that on a particularly clear night. This isn’t a perfect heuristic, but it seems reasonable to start with as a baseline.
Per this calculation[2], 1 lx is approximatly equal to 5 x 10^15 photons per second per square meter. The best moonlight you can typically expect from a full moon is approximately a tenth of this luminosity.
Then per the notes here[3], there are only something like 450 photons per cubic cm in space on average, which seems reasonable for a first approximation of diffused light in interstellar space. That’s vastly lower than 0.1 lx, and that’s before considering that the majority of those photons aren’t actually visible light.
