If you were to stand on the moon, looking towards Earth, could you see the cities on the dark side as spots of light (naked eye)? If so, what would be the ideal conditions for this?
If it’s possible, I’m pretty sure it’s only during a lunar eclipse, since sunlight reflected from an area in daylight or directly from the sun near the earth would outshine everything.
This is perhaps two separate questions.
First, did the Apollo astronauts see lit cities or take pictures of lit cities when they were on the moon?
That answer seems to be no. Putting earth viewed from moon into Google images beings up dozens of pictures of the earth, some of them including the dark earth. None shows any cities. Nothing that small can be seen at all.
The second implied part is whether today’s much better equipment could pick up a tiny spot of light from the moon. That answer I don’t know.
You say naked eye in the OP, so maybe today’s equipment doesn’t matter. But I believe eyes are more sensitive than the film stock used in the Apollo days so I don’t want to rule it out entirely just based on the pictures.
It doesn’t seem likely, though.
What’s a city?
Where does Tokyo-proper end and the megalopolis start?
From the moon, with the naked eye, the Earth is about 2º across which is about twice what the Moon is to us. I would say with very good conditions, with the Earth illuminated by the Sun, continents would be visible.
From the Moon, looking at the dark side of the Earth, we have the problem that we are looking into the Sunlight so my guess is that it would not be possible to see any city lights from the Moon with the naked eye. With a telesope I am sure it must be possible though.
Since there’s no atmosphere on the Moon to create haze, you could look at the Earth through a handy cardboard tube (if we could ignore the need for a pressure suit for a moment) and effectively block out any other light.
However – consider the scale of things. We’ve all seen pictures of the Earth surface at night, with the coasts defined by city lights. But that’s from Low Earth Orbit. If you’re on the Moon, you’re over a thousand times farther away.
Both the facts on my mind have been mentioned already. For one, we have all seen the pretty pictures of Earth at night from LEO. Also, we have seen the Apollo pictures with no such thing. Then again, I was guessing those pictures were exposed for the lit part to look well, not the dark part, so I don’t take those to mean that it is not visible.
The city/megalopolis issue is irrelevant to this question. I don’t care to discern Miami from Fort Lauderdale, I just want to see human lights from the moon.
I understand the problem of the angles involved. To see the dark part, you are either seeing it next to a lot of the bright part or the Sun, so there is always something very bright right next to it to overwhelm it. The cardboard tube idea, though, is not silly at all. The Hubble does it as do many other observatories and satellites. Masking a part of the sky to shield sensitive instruments is a reasonable way to go.
So lets’ say you are looking at a well masked sky (or in an eclipse). Are man made lights too faint to be seen by the naked eye from the moon under ideal circumstances?
If your eyes are dark adapted they are pretty darn sensitive actually.
During a lunar eclipse, where virtually all the earth as seen from the moon is “dark” and the surface of the moon surrounding you is “dark” as well, my semi educated/experienced WAG is that you could see the brighter cities on earth (also assuming the sky above said cities is also fairly clear).
I would have thought it was more to do with inadequate exposure times than “being too small”. In other words, the photos taken on the moon don’t show cities for the same reason they don’t show stars (conspiracy theories notwithstanding). The pictures were taken in daylight, with exposures appropriate to daylight. Maybe the astronauts saw cities, but the photos would not have shown them.
Which is why I explicitly didn’t rule that out.
The problem remains that I don’t recall any statements from the astronauts indicating that they could see cities or lights and plenty of statements that they couldn’t spot any features that small. That’s just my memory speaking so I’m going to hedge that as well, but I would think that any sighting of a lit city from the moon would be prominently featured in all discussions of the Apollo program.
Metropolitan areas are considerably larger and brighter today than 40 years ago so perhaps conditions have changed sufficiently to make it possible. For me the weight of the evidence still says no.
Small, as in angular size as seen from a distance is NOT a problem. If it is angularly small its will just look like a point source, like a star.
Brightness is the problem.
Keep in mind, you can see the light in the night sky on earth from a big city from a LONG way away.
That light is spread over a large fraction of the sky.
That light is just the light scattered by the atmosphere back towards the earthbound country dweller. Most likely significantly more continues out into space (towards the moon for example).
Sure the moon is much farther away from the city than the country dweller is. BUTTTT as seen from the moon, the light one would see would be concentrated into a near point source rather than being spread across most of the sky. My back of the envelope calculation/WAG is those two factors roughly cancel out.
My WAG is the cities would be visibly detectable, but not blaringly bright and obvious either. Which means some real calculations would be required to truelly determine the possibility of visibility.
Also remember, the astronauts were not even probably REMOTELY dark adapted. Thats the same reason they didn’t report seeing many (any?) stars during their surface moon walks. Cities from earth would be on the same level of brightness as stars.
Even in their rocket/capsule on the way too/from the earth, they would not be dark adapted.
Try sitting in a brightly lit room at night. Dash outside and tell me how many stars you can see in the first few seconds or so. You will only see the very brightess ones at best…
I am sure that your typical astronaut, with a job to do in a bit of a rush and totally surrounded by white floor is anything but dark adapted. You do raise an interesting point, though. Has any astronaut said whether they could see stars while on the moon? In forty years of interviews, I am sure this must have come up at some point. Specially if they went to middle schools.
Oh
Another consideration that might make it harder to see city lights from the moon is this.
During an eclipse of the sun by the moon as seen from the Earth, the “black” disk of the Earth is going to be ringed around the edge by the fairly bright atmosphere of the earth that is still in sunlight/has it passing through it.
Now, how bright it would be and whether it would be bright enough and close enough to cause problems seeing the city lights I am not terribly sure about.
thinking about it some more, I imagine it would be a not so bright very red ring.
Seeing this red ring of the earth in a jet black sky filled with many stars and perhaps a few sparkles of the larger/brighter cities would be quite spectacular IMO.
If the alignment was such that the backdrop of the eclipsed sun/earth was a brighter part of the milkyway it would really rock visually.
The “clear sky” part may be more a factor that we think. Using Google image search for “eclipsed Earth” led to some partially eclipsed Earth shots from Time, such as this one.
How bright would a 100 watt light bulb be if viewed from the distance of the moon?
This book contains a starting point:
Now, Alpha Centauri has an apparent magnitude of -.01, so a 100 watt light bulb at 2.5 km will also have a magnitude of -0.01.
The relation between magnitude and distance is taught in astronomy 101:
m - M = 5 X log (d) -5 (d in parsecs, m = apparent magnitude, M = absolute magnitude, which is the magnitude at a standard distance of 10 parsecs)
Plugging the data into this equation will supply the absolute magnitude of a 100 watt bulb, and then allow calculation of the bulb’s apparent brightness at a distance of 384,400 km; the earth to moon distance.
Converting parsecs to km changes the equation to:
m - M = 5 X log (d X 3.24 X 10^-14) -5
for the light bulb:
-0.01 - M = 5 log(2.5 X 3.24 X 10^-14) -5
…
M = 70.45.
plug in M and lunar distance to get apparent magnitude:
m - 70.45 = 5 X log (384,400 X 3.24 X 10^-14) -5
…
m = 25.9
Magnitude 7 is about as dim an object as a dark adapted human can see, however, Hubble, with a magnitude limit of about 30 (see wikipedia) could see a hundred watt bulb at the distance of the moon.
Now, what about cities?
Is a city as bright as 10,000,000 hundred watt light bulbs? A gigawatt is a lot of power. Let’s use that as the brightness of a point source sized city.
Magnitudes and intensity relate by the formula:
m1 - m2 = 2.5 log(I2/I1) (astron 101 again)
so we have
25.9 - m2 = 2.5 log(10000000/1)
…
m2 = 8.4
10,000,000 light bulbs of 100 watts each will only have a massed magnitude of 8.4 when viewed from the distance of the moon.
This is not bright enough for a human to see, but should be within the range of a small telescope, or even binoculars.
to get to magnitude six or so you need another factor of 5 or so more light.
So, make it a large metro area with a larger population, make it a few hundred to a thousand watts of light per person.
Throw in a really nice, dark, and atmosphere free view from the moon and it sounds to me densely populated urban areas would just be visible faintly to a dark adapted human eye.
thanks for the calcs!
Quite so, and it gives me a chance to mention one of my favourite astronomical facts - all the naked-eye visible stars put together don’t cover as much area of sky as Pluto does. (The stars are perfectly visible, despite that.)
Thanks to Squink for the the calcs. Very interesting. Do I gather from billfish678’s post that there might be some visible areas as of now? 100W light bulbs inside a home are somewhat inefficient in lighting the moon, but industrial parks, highways, sports arenas and many other human enterprises are just great at polluting the sky with light.
Also, it might well be that the Earth of the Apollo era was not as bright as it is today (as someone else pointed out earlier). Maybe we weren’t there yet at the time of the lunar landings but we are now.
This is also assuming no atmospheric interference of any sort.
Also no trees, buildings, or light fixtures to block off emission in the direction of the moon.