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View Full Version : Moon, how large would a structure need to be...



Stickrust
2015-Dec-19, 05:18 AM
Hello!

I have bee curious about this for many years now. I was just never sure who to ask.

How large would a man-made structure on the moon need to be for it to be seen by the naked eye on earth?

If a man-made structure on the moon has artificial lights, how much light would it need to be to be seen on earth?

If this has been written about before I would love the link.

Thank you!

Fiery Phoenix
2015-Dec-19, 07:15 PM
To be seen from Earth (i.e. at ~384,0000 km away) is a bit of a tall order. I think you could get a reasonable estimate if you crunch the numbers, but it would probably have to be a megastructure, something the size of a giant mountain, for it to be discernible from here. And even then it wouldn't be much of a sight because of the distance.

DaveC426913
2015-Dec-19, 09:07 PM
It would depend more on contrast than on size.

The Moon's albedo is about 12%, A huge skating rink (albedo ~35%) would be far more visible than a colossal asphalt parking lot (albedo ~10%).

Here's some examples (sadly, Moon and pavement are not represented)
https://en.wikipedia.org/wiki/Albedo#/media/File:Albedo-e_hg.svg
https://en.wikipedia.org/wiki/Albedo

Noclevername
2015-Dec-19, 09:32 PM
According to this (http://www.movoto.com/blog/novelty-real-estate/house-on-moon/), a contrasting color about twice the square footage of New York City would be naked eye visible as a dot.

swampyankee
2015-Dec-20, 12:56 PM
Crunching numbers 8-) ...

According to http://www.atnf.csiro.au/outreach/education/senior/astrophysics/resolution_sensitivity.html, a human eye can resolve about 0.00029 radians,which is little enough for the small angle approximation to work. At 384,000 km, that would mean an object about 100 km across.

antoniseb
2015-Dec-20, 01:20 PM
... a human eye can resolve about 0.00029 radians,which is little enough for the small angle approximation to work. At 384,000 km, that would mean an object about 100 km across.
So actually that is the largest it could be and still only be resolved as a shapeless dot. Is you had a spot with an albedo of 1.00 on the maria with an albedo of about 0.10, it would stand out even if it were considerably less than 100 km. Take for example the case of the bright dot on Ceres. That was observed long before it could be resolved into multiple pixels.

Stickrust
2015-Dec-20, 05:45 PM
Thanks so much everyone! I have always been curious how much humans could change the appearance of the moon for everyone. This helped me put it into perspective. I really appreicate the responses. Thank you.

BigDon
2015-Dec-20, 07:07 PM
Okay, but what if said object, like most human structures, emitted light instead of merely reflecting it?

I'm sure at least one of you fine people know how much light a city the size of San Francisco puts out.

George
2015-Dec-20, 07:51 PM
Crunching numbers 8-) ...

According to http://www.atnf.csiro.au/outreach/education/senior/astrophysics/resolution_sensitivity.html, a human eye can resolve about 0.00029 radians,which is little enough for the small angle approximation to work. At 384,000 km, that would mean an object about 100 km across. Since the Moon is roughly a little over 30 arcminutes in diameter and the human eye can distinguish objects with a resolution of about 1 arcminutes (0.00029 radians), then simply slice the Moon into about 30 slices both ways to get a pixel size for the smallest object distinguishable ignoring brightness.

To antoniseb's point, all stars (Sun excluded but it's too bright to see ;)) are less than one arcminute but we see them fine if they are bright enough.

DaveC426913
2015-Dec-20, 07:59 PM
Since the Moon is roughly a little over 30 arcminutes in diameter and the human eye can distinguish objects with a resolution of about 1 arcminutes (0.00029 radians), then simply slice the Moon into about 30 slices both ways to get a pixel size for the smallest object distinguishable ignoring brightness.
2200/30*1.6= 100km, just as Swampy said.

George
2015-Dec-20, 08:00 PM
Okay, but what if said object, like most human structures, emitted light instead of merely reflecting it? The reflective photons become additive to the emitted ones. We can see some dim objects in daytime because we see both the sky's photons added to the objects photons. "It's all about photons", a club member once said on tv.


I'm sure at least one of you fine people know how much light a city the size of San Francisco puts out. That's an interesting question since I wonder if we would see any city lights from a distant point like the Moon. I don't recall any astronaut claiming they saw a city and they would have had they'd seen one. A bright nucleus of a city seen at a distance that would bring it down to 1 arcminute might be observable, but it diminishes with the inverse square law at farther distances, and the Moon's distance is beyond this point.

George
2015-Dec-20, 08:02 PM
2200/30*1.6= 100km, just as Swampy said. Yes, but perhaps it is easier to think in terms of fractions of the object they are observing. It is hard to imagine 100km on the Moon, but easy to think if 1/30. Both are the same value.

efanton
2015-Dec-20, 09:46 PM
If I recall correctly the Lunar Reconnaissance Oribiter in it latest maps earlier this year was giving a resolution of 50cm per pixel. That's reasonably detailed but you have to consider that this orbiter was fitted with the best camera technically possible before its launch and it is orbiting only 50km above the moons surface.


The google Moon images dont come even close to this resolution. typically, unless they had added high resolution photos for special sites, only an object of 500m in length becomes visible and then only in a very blurred form. This is why ALL conspiracy theories about building or vehicles on the moon are complete nonsense. A building in excess of 1km by 1km would just be a blur or a very small number of pixels, so blurred that it would be almost impossible to claim this 'building' was not just random noise.

Hubble if used, could only resolve objects in excess of 250m across.

From Earth even the very best amateur scopes (16 inch plus), in the very best seeing conditions, could not resolve anything less than 30km to 50km across as far as I can recall

Hornblower
2015-Dec-21, 01:06 AM
If I recall correctly the Lunar Reconnaissance Oribiter in it latest maps earlier this year was giving a resolution of 50cm per pixel. That's reasonably detailed but you have to consider that this orbiter was fitted with the best camera technically possible before its launch and it is orbiting only 50km above the moons surface.


The google Moon images dont come even close to this resolution. typically, unless they had added high resolution photos for special sites, only an object of 500m in length becomes visible and then only in a very blurred form. This is why ALL conspiracy theories about building or vehicles on the moon are complete nonsense. A building in excess of 1km by 1km would just be a blur or a very small number of pixels, so blurred that it would be almost impossible to claim this 'building' was not just random noise.

Hubble if used, could only resolve objects in excess of 250m across.

From Earth even the very best amateur scopes (16 inch plus), in the very best seeing conditions, could not resolve anything less than 30km to 50km across as far as I can recall
My bold. You appear to be mistaken. A 30km feature would subtend about 15 arcseconds, and a good 16-inch scope can resolve under 1/2 arcsecond in good seeing.

Fiery Phoenix
2015-Dec-21, 02:58 AM
Just for reference, this article (http://www.skyandtelescope.com/observing/celestial-objects-to-watch/moon/how-to-see-all-six-apollo-moon-landing-sites/) contains images taken by the Lunar Reconnaissance Orbiter showing Apollo landing sites. They are basically bright-ish dots on the surface. You can just about barely discern them.

efanton
2015-Dec-21, 05:44 AM
From Earth even the very best amateur scopes (16 inch plus), in the very best seeing conditions, could not resolve anything less than 30km to 50km across as far as I can recall


My bold. You appear to be mistaken. A 30km feature would subtend about 15 arcseconds, and a good 16-inch scope can resolve under 1/2 arcsecond in good seeing.


I am most likely wrong. I did say it was from recollection. Maybe it was 3 to 5 km. the 3 and 5 seem to be stuck in mind in relation to what was possible through a scope.

However if you contend that a scope can resolve 0.5 arc second and 30km would be 15 arc seconds then the answer would be in the 1km to 3 km range.

AGN Fuel
2015-Dec-22, 02:16 AM
I used to tell people that my 8" SCT could see objects on the moon to a resolution of around 1 milli-amore. For the few who bothered to ask, the moon has a diameter of around 3500km and it's axiomatic that when the moon hits your eye like a big pizza pie (i.e. when it is full), that's amore.

Hence, a milli-amore is around 3.5km.

DaveC426913
2015-Dec-22, 05:54 AM
BaZINGa! :D

:stops Googling for this term amore: