# Could we illuminate the moon from Earth? **URL:** \ **Category:** Factual Questions\ **Created:** [August 7, 2006, 5:16pm UTC](https://boards.straightdope.com/t/could-we-illuminate-the-moon-from-earth/367714 "2006-08-07T17:16:20Z")\ **Posts on this page:** 20\ **Page:** 1
**Author:** ![psychonaut](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/psychonaut/32/4655_2.png) [@psychonaut](https://boards.straightdope.com/u/psychonaut)\ **Post date:** [August 7, 2006, 5:16pm UTC](https://boards.straightdope.com/t/could-we-illuminate-the-moon-from-earth/367714/1 "2006-08-07T17:16:20Z")
Would it be possible to illuminate the moon from the Earth? For example, say [GoldenPalace.com](http://en.wikipedia.org/wiki/GoldenPalace.com), the online casino notorious for its publicity stunts, decides it wants to advertise on the moon. The idea is to beam light at a new moon such that the company’s logo, clearly visible to the naked eye, is reflected on the surface of the moon. Is this physically and technologically possible to do from the Earth? How about from Earth orbit? From moon orbit? What equipment and strength of lighting would be necessary? Any idea how much it might cost and how much power it would consume? ---
**Author:** ![DirkGntly](https://avatars.discourse-cdn.com/v4/letter/d/d9b06d/32.png) [@DirkGntly](https://boards.straightdope.com/u/DirkGntly)\ **Post date:** [August 7, 2006, 5:41pm UTC](https://boards.straightdope.com/t/could-we-illuminate-the-moon-from-earth/367714/2 "2006-08-07T17:41:57Z")
I don’t know how feasible this would be…probably part of the hardware would break, and you’d wind up with a sign that said, “CHA” Anyway… Making a specific sign might be difficult, but it’s not uncommon to be able to see the dark side of a partial moon behind the lit portion due to “earthshine.” This is not what you’re asking, and I doubt seriously that what you are asking could be done without huge reflectors installed on the surface of the moon. ---
**Author:** ![Squink](https://avatars.discourse-cdn.com/v4/letter/s/b5e925/32.png) [@Squink](https://boards.straightdope.com/u/Squink)\ **Post date:** [August 7, 2006, 6:07pm UTC](https://boards.straightdope.com/t/could-we-illuminate-the-moon-from-earth/367714/3 "2006-08-07T18:07:47Z")
The moon only reflects about 12% of the light hitting it. With [a good sized laser (2.3 watts) and a 3.5 meter telescope](http://physics.ucsd.edu/~tmurphy/apollo/basics.html), it is possible to pick up multiple photons from the old [Apollo lunar ranging retroreflector arrays](http://geophysics.ou.edu/solid_earth/readings/apollo_laser_ranging.html). Those arrays have a reflectivity of over 99%, so no, there’s no practical way for an earthbound group to light up the moon with advertising. ---
**Author:** ![Kozmik](https://avatars.discourse-cdn.com/v4/letter/k/8dc957/32.png) [@Kozmik](https://boards.straightdope.com/u/Kozmik)\ **Post date:** [August 7, 2006, 9:02pm UTC](https://boards.straightdope.com/t/could-we-illuminate-the-moon-from-earth/367714/4 "2006-08-07T21:02:32Z")
But if we could it would be really neat. I want to see the bat signal projected onto the moon. Do you think that it can be technologically achievable to do this in the next century? ---
**Author:** ![groman](https://avatars.discourse-cdn.com/v4/letter/g/73ab20/32.png) [@groman](https://boards.straightdope.com/u/groman)\ **Post date:** [August 7, 2006, 9:02pm UTC](https://boards.straightdope.com/t/could-we-illuminate-the-moon-from-earth/367714/5 "2006-08-07T21:02:34Z")
> [@Squink](#): > > Those arrays have a reflectivity of over 99%, so no, there’s no practical way for an earthbound group to light up the moon with advertising. Are you sure? 12% is damn good. There have been lasers built in the megawatt range, maybe not in the visible spectrum, but still. If you were really determined you could probably make a semi-orbital system to accomplish this. Suppose a lens array in geosynchronous orbit that does the actual writing and disperses the beam so it’s just wide enough when it hits the moon(really fast so it looks like a continuous image for), and a many megawatt laser on earth pointed at the satelite. Very difficult and expensive? Yes. Impossible or impractical? Not really, given most companies’ advertising budgets. ---
**Author:** ![Chronos](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/chronos/32/134_2.png) [@Chronos](https://boards.straightdope.com/u/Chronos)\ **Post date:** [August 7, 2006, 9:15pm UTC](https://boards.straightdope.com/t/could-we-illuminate-the-moon-from-earth/367714/6 "2006-08-07T21:15:42Z")
> [@](#): > > With a good sized laser (2.3 watts) and a 3.5 meter telescope, it is possible to pick up multiple photons from the old Apollo lunar ranging retroreflector arrays. Those arrays have a reflectivity of over 99%, so no, there’s no practical way for an earthbound group to light up the moon with advertising. It gets even worse than that. Not only are the retroreflectors highly reflective, they’re also designed to reflect the light that hits them straight back to where it came from (this is the “retro” part of “retroreflector”). Even if the entire Moon had an albedo that high, most of the light would just be scattered every which way, not back towards Earth. ---
**Author:** ![Squink](https://avatars.discourse-cdn.com/v4/letter/s/b5e925/32.png) [@Squink](https://boards.straightdope.com/u/Squink)\ **Post date:** [August 7, 2006, 10:09pm UTC](https://boards.straightdope.com/t/could-we-illuminate-the-moon-from-earth/367714/7 "2006-08-07T22:09:38Z")
> [@groman](#): > > Are you sure? 12% is damn good. Damn good? That’s about the same as [dark, wet, dirt.](http://en.wikipedia.org/wiki/Image:Albedo-e_hg.png) You can roughly calculate the laser power you’d need by comparing the telescope’s mirror area to the size of the human pupil, and multiplying that by a factor for the reduced reflectivity of the moon relative to a retroreflector. The 3.5 meter scope has an area 25,000,000 times that of a (7 mm) human pupil. A retroreflector will bounce back roughly 8.33 fold more photons than the average piece of lunar surface. 8.33 X 25,000,000 = 208,250,000. Multiply that by the wattage of the scope laser (2.3 W), and you’d need a **480 megawatt laser to be naked eye visible.** Actually you’d need a bigger laser than that because the calculation ignores the geometric factors **Chronos** brought up. ---
**Author:** ![scr4](https://avatars.discourse-cdn.com/v4/letter/s/59ef9b/32.png) [@scr4](https://boards.straightdope.com/u/scr4)\ **Post date:** [August 7, 2006, 10:30pm UTC](https://boards.straightdope.com/t/could-we-illuminate-the-moon-from-earth/367714/8 "2006-08-07T22:30:27Z")
> [@](#): > > …The idea is to beam light at a new moon such that the company’s logo,… The problem with a new moon is that we can’t see it. That’s when the moon is roughly in the same direction as the sun, as seen from earth. So you’d have to use a thin crescent moon. But even then, you are competing with the sunlit part of the moon. Let’s assume that the “writing” needs to be 1/10 as bright as the sunlit part, and you want to illuminate 5% of the moon to that brightness. The moon is about 1700 km in radius, so approximating it as a flat disk, we have 9 million square kilometers that need to be illuminated. Sunlight is about 1400 W/m[sup]2[/sup], so we need 140 W/m[sup]2[/sup] multiplied by 9 million km[sup]2[/sup], which is about 10[sup]15[/sup] watt. That’s one billion megawatts. It takes about 200,000 barrels of oil _per second_ to maintain this illumination, and that’s assuming 100% efficient lamps. ---
**Author:** ![groman](https://avatars.discourse-cdn.com/v4/letter/g/73ab20/32.png) [@groman](https://boards.straightdope.com/u/groman)\ **Post date:** [August 7, 2006, 10:33pm UTC](https://boards.straightdope.com/t/could-we-illuminate-the-moon-from-earth/367714/9 "2006-08-07T22:33:48Z")
> [@Squink](#): > > Multiply that by the wattage of the scope laser (2.3 W), and you’d need a **480 megawatt laser to be naked eye visible.** Actually you’d need a bigger laser than that because the calculation ignores the geometric factors **Chronos** brought up. Point taken. However with [this](http://www.llnl.gov/str/Petawatt.html) you should be able to leave a visible trace on the moon, however, it might be permanent. ---
**Author:** ![Mangetout](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/mangetout/32/19_2.png) [@Mangetout](https://boards.straightdope.com/u/Mangetout)\ **Post date:** [August 7, 2006, 10:53pm UTC](https://boards.straightdope.com/t/could-we-illuminate-the-moon-from-earth/367714/10 "2006-08-07T22:53:38Z")
And these calculations are just to produce a visible _dot_ on the moon; once you start rapidly moving the beam to try to spell out a logo, you’re spreading out the energy over a wider area, so to remain visible, you need even more of it. ---
**Author:** ![MostlyClueless](https://avatars.discourse-cdn.com/v4/letter/m/ba9def/32.png) [@MostlyClueless](https://boards.straightdope.com/u/MostlyClueless)\ **Post date:** [August 7, 2006, 10:56pm UTC](https://boards.straightdope.com/t/could-we-illuminate-the-moon-from-earth/367714/11 "2006-08-07T22:56:10Z")
Blast it, I just need to post this: [![](https://pbfcomics.com/?cid=PBF026AD-Schlorbians_Again.jpg) ](https://pbfcomics.com/?cid=PBF026AD-Schlorbians_Again.jpg) (link to one of the best comics on the web so far… but that’s just my opinion.) ---
**Author:** ![psychonaut](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/psychonaut/32/4655_2.png) [@psychonaut](https://boards.straightdope.com/u/psychonaut)\ **Post date:** [August 7, 2006, 10:58pm UTC](https://boards.straightdope.com/t/could-we-illuminate-the-moon-from-earth/367714/12 "2006-08-07T22:58:08Z")
> [@scr4](#): > > The problem with a new moon is that we can’t see it… So you’d have to use a thin crescent moon. Huh? We’ll be able to see the new moon just fine once we illuminate it (or part of it). The new moon is perfect for beaming a logo onto, since we need only make the surface brighter than near-total darkness, rather than brighter than the sunlight reflected off the surface. Or are you suggesting (surely not!) that because we can’t see the moon, we’ll have no idea where to direct our beam of light? You do realize, I hope, that the moon makes a very regular and predictable path across the sky, and that even a blind man could tell you precisely where it is in the sky (new, full, or otherwise) by doing some calculations or reading off an astronomical table? ---
**Author:** ![Omphaloskeptic](https://avatars.discourse-cdn.com/v4/letter/o/bcef8e/32.png) [@Omphaloskeptic](https://boards.straightdope.com/u/Omphaloskeptic)\ **Post date:** [August 7, 2006, 11:18pm UTC](https://boards.straightdope.com/t/could-we-illuminate-the-moon-from-earth/367714/13 "2006-08-07T23:18:35Z")
> [@psychonaut](#): > > Huh? We’ll be able to see the new moon just fine once we illuminate it (or part of it). The new moon is perfect for beaming a logo onto, since we need only make the surface brighter than near-total darkness, rather than brighter than the sunlight reflected off the surface. Or are you suggesting (surely not!) that because we can’t see the moon, we’ll have no idea where to direct our beam of light? You do realize, I hope, that the moon makes a very regular and predictable path across the sky, and that even a blind man could tell you precisely where it is in the sky (new, full, or otherwise) by doing some calculations or reading off an astronomical table? The new moon actually isn’t very good for this; because it’s close to the sun in the sky, it’s only visible near daylight hours when the sky is fairly bright, so you’ve got a large background level to overcome. Even ignoring the atmospheric light, though, even a new moon’s got a lot of light to overpower. Just from earthlight, the moon receives (if my BOTE calculations are correct) something on the order of 10[sup]-1[/sup] W/m[sup]2[/sup]. The human eye can resolve something on the order of 100 pixels across the moon’s diameter, so this means the new-moon background that your light source has to overpower is something like 10[sup]8[/sup]-10[sup]9[/sup] W/pixel. (Interestingly, this is about the same number as **Squink** came up with using a totally different argument.) Not impossible, but an engineering challenge. ---
**Author:** ![scr4](https://avatars.discourse-cdn.com/v4/letter/s/59ef9b/32.png) [@scr4](https://boards.straightdope.com/u/scr4)\ **Post date:** [August 7, 2006, 11:22pm UTC](https://boards.straightdope.com/t/could-we-illuminate-the-moon-from-earth/367714/14 "2006-08-07T23:22:57Z")
> [@psychonaut](#): > > Huh? We’ll be able to see the new moon just fine once we illuminate it (or part of it). I meant we can’t see it because it’s very close to the sun, and therefore hidden in the glare of the sun. ---
**Author:** ![Bryan\_Ekers](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/bryan_ekers/32/183_2.png) [@Bryan\_Ekers](https://boards.straightdope.com/u/Bryan_Ekers)\ **Post date:** [August 7, 2006, 11:41pm UTC](https://boards.straightdope.com/t/could-we-illuminate-the-moon-from-earth/367714/15 "2006-08-07T23:41:11Z")
What about drawing the Pepsi logo on a moon in eclipse? It might look extra-cool with the corona all around it and junk. ---
**Author:** ![Chronos](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/chronos/32/134_2.png) [@Chronos](https://boards.straightdope.com/u/Chronos)\ **Post date:** [August 8, 2006, 2:56am UTC](https://boards.straightdope.com/t/could-we-illuminate-the-moon-from-earth/367714/16 "2006-08-08T02:56:46Z")
> [@](#): > > What about drawing the Pepsi logo on a moon in eclipse? It might look extra-cool with the corona all around it and junk. Pepsi, not a chance, and they wouldn’t use the rights if they had them, anyway. But 6+ has put together a very realistic proposal, and I hate to say it, but I’m short on funds, and tempted to take them up on it… ---
**Author:** ![Rhubarb](https://avatars.discourse-cdn.com/v4/letter/r/59ef9b/32.png) [@Rhubarb](https://boards.straightdope.com/u/Rhubarb)\ **Post date:** [August 8, 2006, 3:11am UTC](https://boards.straightdope.com/t/could-we-illuminate-the-moon-from-earth/367714/17 "2006-08-08T03:11:35Z")
> [@Chronos](#): > > Pepsi, not a chance, and they wouldn’t use the rights if they had them, anyway. But 6+ has put together a very realistic proposal, and I hate to say it, but I’m short on funds, and tempted to take them up on it… You’re a regular lunatic, Harriman. ---
**Author:** ![Satyagrahi](https://avatars.discourse-cdn.com/v4/letter/s/f475e1/32.png) [@Satyagrahi](https://boards.straightdope.com/u/Satyagrahi)\ **Post date:** [August 8, 2006, 3:16am UTC](https://boards.straightdope.com/t/could-we-illuminate-the-moon-from-earth/367714/18 "2006-08-08T03:16:32Z")
Thank you, lady and gents, for an obscure literary reference that warms my heart… 🙂 –a fellow C.o.H. ---
**Author:** ![The\_Flying\_Dutchman](https://avatars.discourse-cdn.com/v4/letter/t/9fc348/32.png) [@The\_Flying\_Dutchman](https://boards.straightdope.com/u/The_Flying_Dutchman)\ **Post date:** [August 8, 2006, 3:43am UTC](https://boards.straightdope.com/t/could-we-illuminate-the-moon-from-earth/367714/19 "2006-08-08T03:43:01Z")
> [@psychonaut](#): > > Would it be possible to illuminate the moon from the Earth? For example, say [GoldenPalace.com](http://en.wikipedia.org/wiki/GoldenPalace.com), the online casino notorious for its publicity stunts, decides it wants to advertise on the moon. The idea is to beam light at a new moon such that the company’s logo, clearly visible to the naked eye, is reflected on the surface of the moon. Is this physically and technologically possible to do from the Earth? How about from Earth orbit? From moon orbit? What equipment and strength of lighting would be necessary? Any idea how much it might cost and how much power it would consume? Maybe I’m missing something here, but isn’t the moon rather small ? It appears small for sure so I’d expect the logo to reflect off most of the breadth of the moon. That will mean millions of square miles of lighted surface. Is there enough energy available in the US for this? ---
**Author:** ![scr4](https://avatars.discourse-cdn.com/v4/letter/s/59ef9b/32.png) [@scr4](https://boards.straightdope.com/u/scr4)\ **Post date:** [August 8, 2006, 4:23am UTC](https://boards.straightdope.com/t/could-we-illuminate-the-moon-from-earth/367714/20 "2006-08-08T04:23:08Z")
> [@The Flying Dutchman](#): > > Is there enough energy available in the US for this? I thought I showed the calculation already. It takes 200,000 barrels of oil to illuminate it for one second. The Exxon Valdez filled with oil can power it for about 6 seconds. [Next page](https://boards.straightdope.com/t/could-we-illuminate-the-moon-from-earth/367714.md?page=2)