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Let's assume that one day there is a substantial human colony on the Moon. Much later, the Moon becomes terraformed.

I'm trying to imagine when and what humans would first be able to detect with hand-held telescopes or binoculars when viewing the Moon from Earth.

Would they first see the lights of a large city on the night side of the Moon? Would they be able to see a city that is the equivalent of Tokyo, say?

Would they not see anything until substantial terraforming had been done and the Moon became a 'blue planet' like Earth?

How can we calculate the size and relative brightness of human construction that would enable the Earth-bound to actually see something distinguishable on the Moon's surface?

Assumptions

There would be plenty of media coverage of course, but I'm interested in what would be the very first signs that would be visible just by looking up on a clear night, possibly with the aid of an ordinary 21st century type aid such as a hand-held telescope or binoculars.

Please ask for clarifications before answering.

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    $\begingroup$ This is an engineering-specific question, since whoever builds the "thing" will probably anticipate it being visible, and might make it so on purpose. It could also be a self-defeating prophesy as builders try to make the first visible thing on the moon first. It could be a big rectangle of black plastic laid out to be seen. $\endgroup$
    – DWKraus
    Mar 18 at 23:57
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    $\begingroup$ It also raises questions. Technically, the first things visible (with a good telescope & laser illumination) were the corner reflectors left by the Apollo missions: lpi.usra.edu/lunar/missions/apollo/apollo_11/experiments/lrr OTOH, any actual building on the moon would almost certainly be under the surface, for shielding against radiation, micrometeorites, and temperature changes. Perhaps they'd build solar arrays large enough to be visible? $\endgroup$
    – jamesqf
    Mar 19 at 0:29
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    $\begingroup$ Are you interested in the smallest object on the Moon which can be seen from Earth with a pair of ordinary binoculars as anything other than a point of light? (That's about 12 km across.) Or are you interested in how strong a point light source on the Moon needs to be in order to be observable from Earth with a pair of ordinary binoculars? (About 300 million candles if surrounded by complete darkness.) $\endgroup$
    – AlexP
    Mar 19 at 0:36
  • $\begingroup$ @jamesqf: The retroreflectors left by the Apollo and Lunokhod missions are not "visible" from Earth for any reasonable meaning of the word visible. Special detectors can detect a handful of photons returned when the retroreflectors are illuminated by a strong laser pulse. $\endgroup$
    – AlexP
    Mar 19 at 0:37
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    $\begingroup$ @AlexP - If people could look up on a clear night and see what appeared to be a point of light on the night side of a crescent moon and say, "See that dot of light? That's where people are!", that would be enough. $\endgroup$ Mar 19 at 10:19
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You could see Tokyo on the moon with a cheap telescope.

  • So using a cheap telescope I found on amazon, one could get 40 times magnification.
  • The full moon is 1/2 a degree in the sky. Or 30 arc minutes.
  • The human eye can see about 28 arc seconds of resolution.
  • So your basically getting 62 "pixels" of moon for the naked eye in horizontal and vertical direction.
  • Your telescope will multiply this by 40 so ~2400 rows of pixels.
  • The moon has a diameter of 3400km, so your looking at a pixel for every 1500m ish.
  • Tokyo is 90x25km ish. So approximately 60x18 pixels.
  • You will not be able to make out individual buildings or roads, but the difference in Albedo between a city and lunar regolith is likely to be noticable.
    • Even if built out of regolith so the same colour and albedo, the surface normals will be structured rather than random resulting in different light levels in that area depending on the sun angle.
  • When the moon isnt full, the city will be very visible, as the city lights will leak and create a bright glow.
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  • $\begingroup$ So Tokyo will be about 1/2 the size the moon looks to the naked eye, when seen through a 40x telescope? $\endgroup$
    – user253751
    Mar 19 at 13:03
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The first thing on the moon that could be visible from earth is already there.

A mirror.

Will anyone notice, 100 feet away, something else Armstrong left behind? Ringed by footprints, sitting in the moondust, lies a 2-foot wide panel studded with 100 mirrors pointing at Earth: the "lunar laser ranging retroreflector array." Apollo 11 astronauts Buzz Aldrin and Neil Armstrong put it there on July 21, 1969, about an hour before the end of their final moonwalk. Thirty-five years later, it's the only Apollo science experiment still running.

Although it can not be viewed by binoculars, it can be seen by telescopes if one times it right.

But guaranteed, any large solar array would reflect light back to earth when the moon is in the correct position, and the flash would be easily seen by binoculars.

But to actually SEE anything and recognize it, here is what the Hubble telescope could see.

Hubble's 94.5-inch mirror has a resolution of 0.024″ in ultraviolet light, which translates to 141 feet (43 meters) at the Moon's distance. In visible light, it's 0.05″, or closer to 300 feet. Given that the largest piece of equipment left on the Moon after each mission was the 17.9-foot-high by 14-foot-wide Lunar Module, you can see the problem.

So if you are talking a human-made feature, 300 feet would be the minimum size for a very powerful telescope.

If you want binoculars, then to see a covered dome over a hundred km. diameter crater, the minimum would be a good 40x or 50 x binocular would work. You are not going to see a typical lunar habitat, of course, or make out any detail - you will only be able to see that something is there.

Magnification 40x, 50x Binoculars with magnification 40x and 50x are very powerful and produced by such optical companies as Oberwerk, Orion.

They are very expensive BTW. With such powerful optical instruments you can see a big picture of the Moon and craters, even small ones.

But 90X is better

Magnification 90x This powerful instrument with magnification 90x by Oberwerk gives you a huge power and you can see unbelievable picture of the Moon and craters.

But I would recommend image stabilization binoculars.

Of course, something like this Orion GiantView BT-100 ED 90-degree Binocular Telescope would give you a clear view of a domed crater, for sure.

enter image description here

Views of lunar craters to wispy nebulae will take on an almost 3D feel in the GiantView BT-100 ED. The 100mm aperture objective lenses gather 56% more light than 80mm binoculars, so you'll be able to see more objects in the sky with greater clarity. Blockquote

But let us go the reverse - what can we see on Earth from the Moon? Seems to me that if we can see it in one direction, we could see it in the other direction. This article is about using remote sensing of the Earth from the Moon.

The largest technical constraint to observing the Earth from a lunar base is spatial resolution. At the sub-Moon point, the diffraction-limited resolution (R) can be approximated (in km) by R = λ/D where λ is the wavelength (in microns) and D is the telescope diameter (in meters). At visible wavelengths a spatial resolution of 1 km or less requires a 1 meter or larger telescope. Figure 2 shows this relation for three telescope diameters. Blockquote

The chart in this pdf demonstrates that a 0.1 m lens would have a resolution of 30 km of a wavelength of 3.0 microns and a resolution of 5 km. of a wavelength of 0.50 microns.

So obviously we could not see an individual building, or even a city block, on the Earth from the Moon. Likewise, we could not reasonably see it in the other direction, either.

But features bigger than a large city on Earth could be discerned from the Moon using a 100 cm. lens. Mind you, this is just discerning that it is there, not seeing any details about it. Like seeing a blob.

Put in perspective, it is doubtful that a nuclear explosion on Earth would be more than a speck as seen from the Moon through very powerful binoculars.

This article concludes by saying

While the arguments for Earth observations from a lunar observatory are intriguing, it is typically considered unlikely that the advantages outweigh the challenges when viewed insularly. However, as stated by [10], “a lunar astronomy program should complement the earth-orbiting satellite program.” For example, one can easily imagine simultaneous observations of the Earth from instrumentation on the Moon and from geosynchronous Earth orbit (GEO) meteorological satellites in order to provide radiometric crosscalibration between instruments.

Let's take another approach. Can we see a meteor impact on the moon from Earth with the naked eye? Apparently, we can. NASA has been looking carefully at the Moon for meteorite impacts, and has logged some that could be seen by the naked eye, if you happened to be looking at just the right time,

To get an idea of how nontrivial, NASA began counting visible strikes. So far it’s tallied more than 300. The one on March 17 was the biggest so far, ten times brighter than anything seen previously, although nowhere near the hypothetical one-kilometer catastrophe you’re talking about. This rock was more like a foot in diameter and weighed maybe 90 pounds.

Still, it was traveling close to 56,000 miles per hour and had an impact equivalent to five tons of TNT, gouging a crater perhaps 65 feet across. NASA has asked the scientists operating the Lunar Reconnaissance Orbiter, now mapping the moon’s surface, to take a picture of the March 17 crater, and they expect to get around to it later this year.

So an explosion of five tons of TNT would do it. That is some industrial accident. Or a very big mining operation.

However, if you were looking at the moon with an infrared imaging device on your binoculars, you could most certainly see evidence of heat signatures from a reasonably large enough colony, say 5 km.in diameter or so. Seen as a pin-prick of light, perhaps.

So apparently you will have to wait until the moon colony develops a substantial light pollution night time map, or a heat signature, of a major city before we will see it reliably and be able to make anything of it, using binoculars, from Earth.

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    $\begingroup$ Do you have any source saying that the Apollo / Lunokhod retroreflectors are visible from Earth? ("Visible" as in "we can take a picture of them", as opposed to "if we illuminate them with a strong laser pulse we can detect a handful of photons reflected back".) $\endgroup$
    – AlexP
    Mar 19 at 0:58
  • $\begingroup$ @AlexP You should have waited for the complete post, rather than the initial teaser. $\endgroup$ Mar 19 at 2:24
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    $\begingroup$ The text still says that the Apollo / Lunokhod retroreflectors "can be seen by telescopes if one times it right". I didn't know that, and it seems very surprising. Do you have a source for this statement? $\endgroup$
    – AlexP
    Mar 19 at 2:30
  • $\begingroup$ @AlexP Obviously, if astronomers have been using it from Earth, it is visible from telescopes on Earth. The timing is over hours. $\endgroup$ Mar 19 at 2:34
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    $\begingroup$ As far as I know, the way they are using those retroreflectors is to shoot a very powerful laser pulse towards the moon and then measure the time it takes for an ultrasensitive detector to sense the handful of photons which come back. I've never heard of any Earth telescope actually imaging those retroreflectors. You said that this was possible, and I would like a source. $\endgroup$
    – AlexP
    Mar 19 at 2:43

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