Prompted by a casual line in a futuristic sci-fi story I read over the weekend, our (reluctant, self-doubting, anti-) hero is moping around the countryside when he looks up and admires the twinkle of the cities now established on the surface of the Moon.
If there were Earth-sized cities [lets say 1 million, 10 million pop as two data points] and they burned energy and spilled light with the same abandon that we do, would we see these lights from the Earth with the naked eye? And all the permutations please - during the day and night, the lit and unlit parts of the Moon’s surface?
Can a person on the moon see Earth cities? The famous “Earthrise” photo taken in 1968 does not show any lights on the dark side, but I don’t know if photo resolution and other issues might be factors in that. Did the moon-landing astronauts remark on this? What about any of the various robot landers that have been there since?
If someone on the moon can see Earth cities, it seems likely that vice versa would be true, allowing for the blurring effects of Earth’s atmosphere on objects in space to make some small difference between the two.
Buzz Aldrin saw flashes on the Earth’s surface that he speculated was artificial light but was later determined to be caused by cosmic rays. Only Apollo 14 attempted to take pictures of Earth’s night side, but it was spoiled by defused light from the day side.
Although resolution may be a factor, exposure is the real reason. Exposed for full daylight, no film or digital system could detect the vastly dimmer lights of a city at night.
Even looking by eye from the moon, I suspect you’d need to have a nearly complete “new earth”/full moon to see city lights on the dark side of the earth. Even then, the light of the sun on you and your surroundings might make it hard to see them. You might need a lunar eclipse.
One of our local physicists will be along shortly to explain exactly how wrong I am.
There’s a couple of issues to think about. Earth has an atmosphere, but more importantly, Earth is naturally quite bright. It reflects light about 4.5 times better than the Moon does. So artificial light on the Moon would stand out against a darker background than artificial light on the Earth does.
The unshielded sunlight, as well as that reflected from the high albedo regolith, would definitely saturate the human eye’s potential to see artificial light emitted from Earth, notwithstanding that an astronaut would have all filters on to protect their vision. Using a sunshield that blocked out all light and facing the full backside of the Moon it might just be possible for the eye to make out light emitted from the largest urban centers.
And while it was the premise of the o.p. that Lunar cities would “…[burn] energy and [spill] light with the same abandon that we do…” in reality cities or settlements on the Moon will almost certainly be buried under the regolith or located in the large lava tubes to protect them from both thermal fluctuations and high energy cosmic radiation, as well as holding sufficient pressure with safety to support unprotected residents.
It might be possible to see them indirectly. At full Moon, it’s often possible to see prominent lunar craters such as Copernicus and Tycho with the naked eye, owing to the ray systems centred on these craters. If a city were located there (as is the case in many sci-fi novels), it’d be easy for an Earth-based observer to make out the location of these cities. But seeing the artificial light emitted by these cities itself would, of course, be another matter.