A future nation wants to make the Moon as bright as possible in order to save massive amounts of night-time electricity on Earth at certain phases of the moon.

The plan is to coat the Moon's surface with a layer of talcum powder. (EDIT or other white powdery substance if talc is unstable on the Moon)


Ignoring the logistics of the exercise, and the availability of the white substance, how bright could the sunny side of the moon be and how bright would the night side be compared to now?

Please ask for clarifications before answering.

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    $\begingroup$ I would be worried about stability of talc Mg3Si4O10(OH)2 in lunar environment its a hydrate. Why not titanium dioxide? $\endgroup$ Mar 15, 2021 at 20:33
  • $\begingroup$ @Gault Drakkor - My thought was that talc is plentiful. Is TiO ₂ easily sourced? Would talc lose its whiteness if it degraded under sunlight? $\endgroup$ Mar 15, 2021 at 20:54
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    $\begingroup$ For the purposes of the question, it would be easy enough to include a bullet that says, "ignore any reason you might think of that would prohibit talcum powder from being used for this purpose." While there might be some excellent reasons why talcum powder wouldn't be the best chemical to use - the vast, vast majority of readers would never be aware of those facts (and those who are will suspend their disbelief for a good story...). Adding that you're using it because it's abundant would also be good, that way anyone wanting to attempt a Frame Challenge would know why you wanted it. $\endgroup$
    – JBH
    Mar 15, 2021 at 23:10
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    $\begingroup$ The irony of the question is that the areas that would receive the most benefit are the areas where there is currently absolutely no night time artificial light. Cities with extensive light pollution would receive minimal benefit. It would be akin to using a flashlight on a brightly lit road in the city but in the country it would perhaps eliminate the need for a flashlight. $\endgroup$ Mar 16, 2021 at 0:22
  • $\begingroup$ Note quite a duplicate, but closely related: Moonlight bright enough to see by?. $\endgroup$ Mar 16, 2021 at 11:56

4 Answers 4


The moon's albedo is around 0.1, meaning it reflects about 10% of the sunlight that hits it, for a peak illuminance of a few tenths of a lux (sources on albedo and lux). Even if the moon were a perfect mirror, it would only reflect 10x as much light and would only cast a few lux, making its illuminance during a full moon only a few hundredths of a percent of the sun. A perfect mirror supermoon would barely qualify as emergency lighting, which is normally a few lux. Under the absolute best conditions, a completely reflective moon would have about a third of the illuminance of a typical streetlamp. On average, it will be less than a tenth of a typical streetlamp, making it rather unsuitable as a replacement for electric lights.

Talcum powder will increase the albedo of the moon, but certainly not as much as a mirror finish, so the effect will be extremely modest at best. No amount of increase in the moon's albedo will make it a viable night-time illumination source that can replace electric lighting on its own, as it will never be as bright as a regular street lamp, which is normally 10 lux. Increasing the albedo of the moon will help you see better during the night, but for almost any application where you already have night-time illumination (streets, sidewalks, outdoor events, etc.), you'll still need illumination even with a 10x as bright moon. The electricity savings will be small, since there is very little external lighting that becomes unnecessary with a brighter moon (although I suppose you could turn all the electric lights down a few percent if the moon makes up the difference).

EDIT: To summarize the comment chain below, I'm not in any way arguing that the moon is not or could not be a useful illumination source at night. But it will never replace electric lighting, as its illuminance is far below that of electric lights. At best, the moon could produce about 10% of the lighting level that is deemed useful and appropriate for the general public in day-to-day life. Since we're talking about a society that is engaging in planetary-scale projects, I think it's safe to assume its citizens need night-time lighting for more than not tripping over things in the dark (which can be done at mere thousandths of a lux, no moon alterations needed). Anything that requires moderate visual acuity, like making out text or faces or objects quickly or at a distance or while moving will be difficult by moonlight no matter how shiny the moon is.

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    $\begingroup$ If you cover the whole surface of the moon with aimable flat mirrors, you could direct all the incident sunlight onto the Earth. (Earth from any point on the moon is much wider than the sun, so despite the sun not being a point source you can ensure the entire reflection hits somewhere on the Earth. This could give illumination anywhere from about 1/4 sunlight down to 1/20th sunlight, depending on your position and angle relative to the moon. But yeah... not what the OP is asking for, and that many mirrors might be slightly expensive $\endgroup$
    – PcMan
    Mar 15, 2021 at 20:55
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    $\begingroup$ Depends on what you use the illumination for. It has been a viable night-time illumination for millennia. Just very limited. $\endgroup$
    – Mary
    Mar 16, 2021 at 1:23
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    $\begingroup$ Full moon being used for illumination? One famous example is the raid by 617 squadron on the Ruhr dams in Germany. The date May 16th, 1943 was specifically chosen because the moon was full that night. bbc.com/news/uk-england-lincolnshire-22481152 $\endgroup$
    – dgnuff
    Mar 16, 2021 at 8:47
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    $\begingroup$ "No amount of increase in the moon's albedo will make it a viable night-time illumination source on its own". The moon already works very well as nighttime illumination. Have you never been outside on a full moon? On a clear night, you can even read a book outside in moonlight. $\endgroup$
    – Polygnome
    Mar 16, 2021 at 9:47
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    $\begingroup$ beware of increasing the moon's energy output toward Earth too much. You will exacerbate the global warming. And confuse all animals. And starve those who relied on darkness to hunt. And... $\endgroup$ Mar 16, 2021 at 10:05

First of all, for lunar regolith we can neglect reflection and just consider scattering.

This paper simulated the scattering properties of the lunar surface, and came to values around 22% of reduced reflectance for incidence angles between 32 and 57 degrees.

scattering properties of lunar regolith

I haven't been able to find a comparable chart for talcum, beyond generic statement like "talcum is highly reflective" or non normalized spectra, like this one

enter image description here

If we make an assumption that talc is from 2 to 3 times more effective than regolith at scattering light, we get that the talc covered lunar surface would roughly reflect back a similar proportion of light.

Since, according to this

Our Moon’s average visual albedo is 0.12. [...] The full Moon as seen on Earth has a visual magnitude of –12.7

and considering that

A difference of 1.0 in magnitude corresponds to a brightness ratio of $(100)^{1/5}$ or about 2.512

the talc covered full Moon should be somewhere around -13.7.

  • $\begingroup$ Simply looking at scattering is insufficient. You have to look at the scattering distribution function of talc vs lunar regolith. The light has to not only be scattered, but scattered in the direction of the Earth. Lunar regolith is actually relatively black, so the moon is a terrible reflector and a poor scatterer. That would be very different if talc were used. $\endgroup$
    – stix
    Mar 16, 2021 at 22:28

Use your talc for mirrors!

First, contemplate the economics of a future nation. Given that it's the future (not necessarily very far in the future), there is one man, the Consumer, who owns all the corporations, minerals, rockets, politicians, religious and charitable foundations, eyeballs, etc. Provided he is happy you have more or less 100% market penetration and you can count the project as complete.

Now talc can make flat tabular crystals. These are rare and usually microscopic, though you can find some pretty alleged examples online in an image search (the better looking ones I found were temporary images from auctioned minerals). If you have the resources to resurface the Moon, I imagine converting the silicon dioxide and magnesium from regolith and scarce lunar water into perfectly flat reflective plates of talc, though as some say, there may be better choices.

Once you have covered the Moon in mobile reflective mirrors, pointing at the Sun, someone looking up at it from the right spot will see lots of Sun, depending on the gaps between mirrors based on the angle they need to make. The "right spot" is actually pretty big, because these mirrors are flat, so anyone within 0.25 degrees (angular radius of the Sun) of the Consumer, as seen from the Moon, will have the same effect. The angular diameter of the Earth is 1.8 to 2.0 degrees, so luckless peasants far from the Capital of the Earth, or who are not invited when the Leader goes on vacation, will see only a drab Moon, lacking in features as the Man in the Moon has been banished as the backward superstition he is. Perhaps some advertising might be superimposed on the space. The transition from this to the lit moon could be very abrupt if the mirrors are all directed correctly, though it might take some doing to correct for small astronomical aberrations based on distance.

Note that this system does not actually focus light, so it can never be brighter than the Sun, though one supposes a wise Leader would have a very well secured feature to permit the mirrors to be warped a little and focused on a potential source of dissent.


According to the Wikipedia (https://en.wikipedia.org/wiki/Moon), the moon has an albedo of 13.6%, which means that 13.6% of the incident light is reflected/scattered by its surface back into space. As such, the max. theoretical gain is a factor of 1/13.6% = 7.4.

That said, to achieve this brightness increase, you must use a very white substance that you can easily create on the moon itself. Otherwise, the logistics of bringing all the megatons of material to the moon would kill you. So, what is a good material? I believe the answer is magnesium oxide: This is a very white powder, so white in fact that it has been used as a reference white for quite some time, as well as in applications demanding for the highest possible albedos possible. Now, the lunar regolith (the dust that covers the moon) consists of a whole bunch of oxygen, as well as 5% magnesium. "All" you need to do is to take a pile of lunar regolith, extract the magnesium and a small part of the oxygen, and produce that awesome white powder to cover the moon's surface with. 1kg of regolith is enough to produce more than 80g of magnesium oxide, which can cover quite a substantical area as a layer of thin, fine dust.


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