# Corner reflector on the moon thought experiment **URL:** \ **Category:** Factual Questions\ **Created:** [November 25, 2024, 6:52pm UTC](https://boards.straightdope.com/t/corner-reflector-on-the-moon-thought-experiment/1010771 "2024-11-25T18:52:41Z")\ **Posts on this page:** 20\ **Page:** 1
**Author:** ![sbright33](https://avatars.discourse-cdn.com/v4/letter/s/8e8cbc/32.png) [@sbright33](https://boards.straightdope.com/u/sbright33)\ **Post date:** [November 25, 2024, 6:52pm UTC](https://boards.straightdope.com/t/corner-reflector-on-the-moon-thought-experiment/1010771/1 "2024-11-25T18:52:41Z")
Please correct me if I’m wrong with Facts. All we need is a corner reflector on the moon. 3 mirrors glued together so they are perpendicular. 3 sides of a cube. The other 3 are open. Like a radar reflector on a ship. It is not perfect but nearly so, such that a perfect powerful red laser focused at that distance reflects back to the earth with a dot a few feet away from where I’m standing. You would calibrate this error by measuring the x,y distance, with respect to the reflector, at the north pole, or when the moon is near the horizon. Your target would be perpendicular to the moon. It’s not so hard as it seems because the cube would not rotate, as the same side of the moon is always facing us. You would expect this x,y offset to remain for awhile. So instead let’s just make it a perfect cube always coming back to exactly the laser source. This is easier to imagine. But would it come back on the same line exactly? Please excuse any math errors. It takes 2.5s for light to travel to the moon and back. During that time the equator rotates 1160m. So the dot would be 1160m to the West of where it started. Am I correct so far? There’s more. Of course the moon would be straight up. ---
**Author:** ![Munch](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/munch/32/5281_2.png) [@Munch](https://boards.straightdope.com/u/Munch)\ **Post date:** [November 25, 2024, 6:56pm UTC](https://boards.straightdope.com/t/corner-reflector-on-the-moon-thought-experiment/1010771/2 "2024-11-25T18:56:23Z")
[List of retro reflectors on the moon](https://en.m.wikipedia.org/wiki/List_of_retroreflectors_on_the_Moon). > [@sbright33](#): > > But would it come back on the same line exactly? Sending any light source to the Moon’s retroreflectors is going to be dispersed in a cone shape - and it’s going to come back via another cone shape that covers the ground lost due to Earth’s rotation. ---
**Author:** ![Darren\_Garrison](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/darren_garrison/32/92_2.png) [@Darren\_Garrison](https://boards.straightdope.com/u/Darren_Garrison)\ **Post date:** [November 25, 2024, 6:56pm UTC](https://boards.straightdope.com/t/corner-reflector-on-the-moon-thought-experiment/1010771/3 "2024-11-25T18:56:24Z")
> **[List of retroreflectors on the Moon](https://en.wikipedia.org/wiki/List_of_retroreflectors_on_the_Moon)** > > Retroreflectors are devices which reflect light back to its source. Six were left at six sites on the Moon by three crews of the Apollo program, two by remote landers of the Lunokhod program, and one by the Chandrayaan program. Lunar reflectors have enabled precise measurement of the Earth–Moon distance since 1969 using lunar laser ranging. > Several unsuccessful attempts to land retroreflectors on the lunar surface have been made, and several future attempts are planned. Ninja’d, so I’ll go more detailed: > **[Laser Beams Reflected Between Earth and Moon Boost Science - NASA](https://www.nasa.gov/missions/laser-beams-reflected-between-earth-and-moon-boost-science/)** > > Dozens of times over the last decade NASA scientists have launched laser beams at a reflector the size of a paperback novel about 240,000 miles (385,000 ---
**Author:** ![Marvin\_the\_Martian](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/marvin_the_martian/32/2898_2.png) [@Marvin\_the\_Martian](https://boards.straightdope.com/u/Marvin_the_Martian)\ **Post date:** [November 25, 2024, 7:00pm UTC](https://boards.straightdope.com/t/corner-reflector-on-the-moon-thought-experiment/1010771/4 "2024-11-25T19:00:04Z")
More details on the Apollo retroreflectors: > **[Laser Ranging Retroreflector](https://en.wikipedia.org/wiki/Laser_Ranging_Retroreflector)** > > The Laser Ranging Retroreflector (LRRR) is the first ever deployable lunar laser ranging experiment. It was carried on Apollo 11 as part of the Early Apollo Scientific Experiments Package, and on Apollo 14 and Apollo 15 as part of the Apollo Lunar Surface Experiments Package (ALSEP). The LRRR consists of a series of corner reflectors set within a panel. Laser beams sent from Earth are bounced off the retroreflector and the timing of the return signal can be used to measure the distance from th... ---
**Author:** ![sbright33](https://avatars.discourse-cdn.com/v4/letter/s/8e8cbc/32.png) [@sbright33](https://boards.straightdope.com/u/sbright33)\ **Post date:** [November 25, 2024, 7:01pm UTC](https://boards.straightdope.com/t/corner-reflector-on-the-moon-thought-experiment/1010771/5 "2024-11-25T19:01:40Z")
I agree with all that about cone shaped. It’s a thought experiment. A perfect laser. ---
**Author:** ![Horatius](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/horatius/32/3572_2.png) [@Horatius](https://boards.straightdope.com/u/Horatius)\ **Post date:** [November 25, 2024, 7:05pm UTC](https://boards.straightdope.com/t/corner-reflector-on-the-moon-thought-experiment/1010771/6 "2024-11-25T19:05:24Z")
> [@sbright33](#): > > But would it come back on the same line exactly? Please excuse any math errors. It takes 2.5s for light to travel to the moon and back. During that time the equator rotates 1160m. So the dot would be 1160m to the West of where it started. Am I correct so far? There’s more. In practical terms, t[he optics aren’t nearly perfect enough to make that matter](https://en.wikipedia.org/wiki/Lunar_Laser_Ranging_experiments): > At the Moon’s surface, the beam is about 6.5 kilometers (4.0 mi) wide By the time it gets back to Earth, it would be even wider, so movement of about a kilometer won’t matter so much. But there’s also this: > The reflected light is too weak to see with the human eye. Out of a pulse of 3×1017 photons[[25]](https://en.wikipedia.org/wiki/Lunar_Laser_Ranging_experiments#cite_note-26) aimed at the reflector, only about 1–5 are received back on Earth, even under good conditions.[[26]](https://en.wikipedia.org/wiki/Lunar_Laser_Ranging_experiments#cite_note-27) This isn’t something you’d whip up in your backyard. You’d need really good telescopes and detectors to accurately detect 5 photons. ---
**Author:** ![sbright33](https://avatars.discourse-cdn.com/v4/letter/s/8e8cbc/32.png) [@sbright33](https://boards.straightdope.com/u/sbright33)\ **Post date:** [November 25, 2024, 7:08pm UTC](https://boards.straightdope.com/t/corner-reflector-on-the-moon-thought-experiment/1010771/7 "2024-11-25T19:08:37Z")
I understand. Please imagine this perfect laser beam. In theory. ---
**Author:** ![Horatius](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/horatius/32/3572_2.png) [@Horatius](https://boards.straightdope.com/u/Horatius)\ **Post date:** [November 25, 2024, 7:10pm UTC](https://boards.straightdope.com/t/corner-reflector-on-the-moon-thought-experiment/1010771/8 "2024-11-25T19:10:07Z")
Well, then, yes, it would be a factor, but just one amongst many. From that page: > To compute the lunar distance precisely, many factors must be considered in addition to the round-trip time of about 2.5 seconds. These factors include the location of the Moon in the sky, the relative motion of Earth and the Moon, **Earth’s rotation** , [lunar libration](https://en.wikipedia.org/wiki/Lunar_libration), [polar motion](https://en.wikipedia.org/wiki/Polar_motion), [weather](https://en.wikipedia.org/wiki/Weather), speed of light in various parts of air, propagation delay through [Earth’s atmosphere](https://en.wikipedia.org/wiki/Atmosphere_of_Earth), the location of the observing station and its motion due to [crustal motion](https://en.wikipedia.org/wiki/Plate_tectonics) and [tides](https://en.wikipedia.org/wiki/Tide), and [relativistic effects](https://en.wikipedia.org/wiki/Theory_of_relativity).[[20]](https://en.wikipedia.org/wiki/Lunar_Laser_Ranging_experiments#cite_note-20)[[21]](https://en.wikipedia.org/wiki/Lunar_Laser_Ranging_experiments#cite_note-21) ---
**Author:** ![sbright33](https://avatars.discourse-cdn.com/v4/letter/s/8e8cbc/32.png) [@sbright33](https://boards.straightdope.com/u/sbright33)\ **Post date:** [November 25, 2024, 7:15pm UTC](https://boards.straightdope.com/t/corner-reflector-on-the-moon-thought-experiment/1010771/9 "2024-11-25T19:15:39Z")
> [@Horatius](#): > > location of the Moon in the sky, the relative motion of Earth and the Moon, **Earth’s rotation** , [lunar libration](https://en.wikipedia.org/wiki/Lunar_libration), [polar motion](https://en.wikipedia.org/wiki/Polar_motion), [weather](https://en.wikipedia.org/wiki/Weather), speed of light in various parts of air, propagation delay through [Earth’s atmosphere](https://en.wikipedia.org/wiki/Atmosphere_of_Earth), the location of the observing station and its motion due to [crustal motion](https://en.wikipedia.org/wiki/Plate_tectonics) and [tides](https://en.wikipedia.org/wiki/Tide), and [relativistic effects](https://en.wikipedia.org/wiki/Theory_of_relativity) Most of these won’t matter. We are measuring the location of the dot, not the precise time. If we could do this experiment it would be close to the results we predicted. ±1% ---
**Author:** ![RitterSport](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/rittersport/32/6326_2.png) [@RitterSport](https://boards.straightdope.com/u/RitterSport)\ **Post date:** [November 25, 2024, 7:16pm UTC](https://boards.straightdope.com/t/corner-reflector-on-the-moon-thought-experiment/1010771/10 "2024-11-25T19:16:07Z")
> [@Horatius](#): > > This isn’t something you’d whip up in your backyard. You’d need really good telescopes and detectors to accurately detect 5 photons. What if you tried [more power](https://what-if.xkcd.com/13/)? (Sort-of on-topic) ---
**Author:** ![sbright33](https://avatars.discourse-cdn.com/v4/letter/s/8e8cbc/32.png) [@sbright33](https://boards.straightdope.com/u/sbright33)\ **Post date:** [November 25, 2024, 7:24pm UTC](https://boards.straightdope.com/t/corner-reflector-on-the-moon-thought-experiment/1010771/11 "2024-11-25T19:24:33Z")
We can actually do the next parts of my experiment! Maybe. First let’s agree that I’m on track so far. ---
**Author:** ![LSLGuy](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/lslguy/32/5813_2.png) [@LSLGuy](https://boards.straightdope.com/u/LSLGuy)\ **Post date:** [November 25, 2024, 7:43pm UTC](https://boards.straightdope.com/t/corner-reflector-on-the-moon-thought-experiment/1010771/12 "2024-11-25T19:43:43Z")
It’s totally unclear to me what the OP is actually asking. If you assume a perfect laser beam with zero spread or loss, and you assume a perfect retro-reflector with zero spread or loss, and you assume no atmosphere to add spread or loss or diffraction or whatever, then … What are you asking about? Does the beam return to Earth? Yes. Does it return to the same spot on Earth? No. For the big reason you mentioned, Earth’s rotation during the transit time, and then for another dozen minor reasons because you haven’t magically assumed them out of existence with your perfect this, that, and the other thing, but not magic perfect _every_thing. So what are you asking about? ---
**Author:** ![Dewey\_Finn](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/dewey_finn/32/4222_2.png) [@Dewey\_Finn](https://boards.straightdope.com/u/Dewey_Finn)\ **Post date:** [November 25, 2024, 7:47pm UTC](https://boards.straightdope.com/t/corner-reflector-on-the-moon-thought-experiment/1010771/13 "2024-11-25T19:47:45Z")
I probably don’t understand the question but does the OP know that the reflected beam is offset from the original beam even if you’re directly in front of the retroreflector? ---
**Author:** ![sbright33](https://avatars.discourse-cdn.com/v4/letter/s/8e8cbc/32.png) [@sbright33](https://boards.straightdope.com/u/sbright33)\ **Post date:** [November 25, 2024, 7:48pm UTC](https://boards.straightdope.com/t/corner-reflector-on-the-moon-thought-experiment/1010771/14 "2024-11-25T19:48:56Z")
It’s a thought experiment. I am only asking about the thing, much bigger than everything else, causing the dot to move from it’s origin. Thank you for your answer LSLguy. You’ll be happy to know I’m coming back to reality in the next part. Had to overcome some theory first to understand the next bit. ---
**Author:** ![sbright33](https://avatars.discourse-cdn.com/v4/letter/s/8e8cbc/32.png) [@sbright33](https://boards.straightdope.com/u/sbright33)\ **Post date:** [November 25, 2024, 7:50pm UTC](https://boards.straightdope.com/t/corner-reflector-on-the-moon-thought-experiment/1010771/15 "2024-11-25T19:50:43Z")
> [@Dewey\_Finn](#): > > I probably don’t understand the question but does the OP know that the reflected beam is offset from the original beam even if you’re directly in front of the retroreflector? So True! Much less than 1100m. Depends on the width of the reflector right? ---
**Author:** ![RitterSport](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/rittersport/32/6326_2.png) [@RitterSport](https://boards.straightdope.com/u/RitterSport)\ **Post date:** [November 25, 2024, 8:37pm UTC](https://boards.straightdope.com/t/corner-reflector-on-the-moon-thought-experiment/1010771/16 "2024-11-25T20:37:59Z")
Probably also depends on where you are on Earth. At the equator, the return beam will be offset a lot more than it would be at the poles. If you were at the rotational north pole, could you send the perfect beam and have it come back to you? ---
**Author:** ![sbright33](https://avatars.discourse-cdn.com/v4/letter/s/8e8cbc/32.png) [@sbright33](https://boards.straightdope.com/u/sbright33)\ **Post date:** [November 25, 2024, 11:15pm UTC](https://boards.straightdope.com/t/corner-reflector-on-the-moon-thought-experiment/1010771/17 "2024-11-25T23:15:00Z")
Yes! That’s how we calibrated the imperfect cube reflector. ---
**Author:** ![CalMeacham](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/calmeacham/32/35_2.png) [@CalMeacham](https://boards.straightdope.com/u/CalMeacham)\ **Post date:** [November 25, 2024, 11:39pm UTC](https://boards.straightdope.com/t/corner-reflector-on-the-moon-thought-experiment/1010771/18 "2024-11-25T23:39:34Z")
> [@sbright33](#): > > I agree with all that about cone shaped. It’s a thought experiment. A perfect laser. Perhaps you don’t realize this, but even a “perfect laser” spreads out. Once you get beyond a distance called the _confocal parameter_ (or half the _Rayleigh Range_) from the laser’s smallest point – its _beam waist_ the beam spreads out at a constant angle, making that “cone shape” people have brought up. Lasers don’t produce narrow, line-like beams forever. Get far enough away and it’s a cone. A pretty wide one by the time you get to the moon. So any offset is going to be lost in the width of any real laser beam. A Gaussian Beam is as close to a “perfect laser” as you’re going to get > **[Gaussian beam](https://en.wikipedia.org/wiki/Gaussian_beam)** > > In optics, a Gaussian beam is an idealized beam of electromagnetic radiation whose amplitude envelope in the transverse plane is given by a Gaussian function; this also implies a Gaussian intensity (irradiance) profile. This fundamental (or TEM00) transverse Gaussian mode describes the intended output of many lasers, as such a beam diverges less and can be focused better than any other. When a Gaussian beam is refocused by an ideal lens, a new Gaussian beam is produced. The electric and magnetic ... ---
**Author:** ![sbright33](https://avatars.discourse-cdn.com/v4/letter/s/8e8cbc/32.png) [@sbright33](https://boards.straightdope.com/u/sbright33)\ **Post date:** [November 26, 2024, 1:02am UTC](https://boards.straightdope.com/t/corner-reflector-on-the-moon-thought-experiment/1010771/19 "2024-11-26T01:02:39Z")
Maybe you’re right. I don’t want to debate this. The exact possible size of the dot is not relevant. Even if the cone is 100 miles across, it is brighter in the center, since it is a gaussian beam. Measurably so. This is not my point. [https://chatg.pt/small23m](https://chatg.pt/small23m) ---
**Author:** ![Horatius](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/horatius/32/3572_2.png) [@Horatius](https://boards.straightdope.com/u/Horatius)\ **Post date:** [November 26, 2024, 1:54am UTC](https://boards.straightdope.com/t/corner-reflector-on-the-moon-thought-experiment/1010771/20 "2024-11-26T01:54:02Z")
> [@sbright33](#): > > This is not my point So what is your point? This experiment has actually been done, several times. What do you hope to achieve that hasn’t been done before? [Next page](https://boards.straightdope.com/t/corner-reflector-on-the-moon-thought-experiment/1010771.md?page=2)