I'm always interested in how much can be figured out from how little. The recent gravitational wave observation, and the reconstruction of its source, is a perfect example. Earthshine is the illumination of the dark part of the Moon by the Earth. How bright it is depends on the phase of the Earth as seen from the Moon, how much of the illuminated part of the Earth that's visible from the Moon is cloud-covered, how much is ice-covered, how much is cloud-free ice-free land, and how much is cloud-free ice-free ocean. Also, its color depends on how what's doing the reflecting (clouds, ice, water, desert land, vegetation, etc.). My question is how good a map of the Earth it would be possible to construct from careful measurements of earthshine from one location on Earth. Assume its brightness can be accurately measured even in the daytime (or explain why it can't be). Note that the Moon's latitude as well as its longitude varies considerably, so it doesn't always have an equatorial view. Decades of observations can be combined so as to average out the clouds and concentrate on long-term features. I'm envisioning an alternate history in which photometry, advanced math, and computers were developed before world travel. (And in fact, the nature of earthshine was understood long before Columbus.) Also, a hypothetical skeptic who wonders if other continents really exist as depicted on maps, and only trusts his own observations, given that computers, telescopes, and photometers are cheaper than world travel. (One final aside on moonlight and gravitational waves: Has anyone else noticed that the peak flux of the gravitational wave event, GW150914, on Earth was seven times the flux of the light of the full Moon? If it was visible light, you could not only have seen it, but could have read by its light. LIGO isn't really very sensitive!)