Talk:Lunar Laser Ranging experiment

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Science results?[edit]

Can this article include a section on the science results? What was found? is it significant? What interesting things have been found out? e.g. tidal forces? magnetic dragging? earthquakes? solar wind effects? General relativity results? ??? linas 00:47, 30 October 2005 (UTC)

I whole heartedly agree. Behold, and marvel in the splendor of Wiki! -Mr.Logic 03:21, 15 January 2006 (UTC)

The statement

- Einstein's theory of gravity and the general theory of relativity predict the moon's orbit to within the accuracy of the laser ranging measurements.

leads to a followup-question: Do Newtonian mechanics not? I mean, that fact is an interesting piece of support for relativity theory only if it's not only due to relativity theory yielding almost the same answer as Newtonian mechanics.

- Theoretically, no. Much in the same way that the perihelion of Mercury precesses about the Sun due to General Relativity, you'd expect the Moon to do the same about the Earth. I don't know whether this effect would really be measurable in the Earth-Moon system using LRRR. Anyone have a calculator?  :-)

On a different note, I found the phrase "The moon is spiraling away from Earth at a rate of 3.8 cm per year, due to the Earth's ocean tides" very interesting. This makes it sound like the moon is moving away because of the tides, which is kind of accurate and kind of not accurate. It's not really the tides themselves that create this effect, but rather the recoil effect of the ocean tides on the land masses that cause the problem. If the Earth had no continents and the ocean depth was uniform then this wouldn't be an issue. So you can't really blame the tides directly.

Toddbu 10:06, 8 April 2006 (UTC)toddbu

Please add this to the article if you can explain it concisely and what exactly is the cause - as a layman I don't understand your reference to the recoil effect. Tempshill 15:54, 29 August 2007 (UTC)
Toddbu, I think you are mistaken. I am under the impression that tidal locking is the result of the tidal bulges being dragged around Earth by the friction between the bulges and the Earth's spin. Since Earth rotates in the same direction the Moon orbits, but faster, it tends to drag the tidal bulges 'ahead' of the Moon, which results in a displaced center of mass, which creates the forward acceleration on the Moon, which causes it to recede from us. This also has the effect of slowing Earth's rotation slightly, given enough time it would result in Earth always facing the same side of the moon. This external link to hyperphysics claims the bulges lead by about 3°. I'll try to work some of this into the article when I get a chance. Quietly (talk) 14:02, 24 July 2008 (UTC)

Lunokhod in NSSDC[edit]

The article previous contained the following:

(Note that NASA's NSSDC catalog confuses Lunokhod 1 and Lunokhod 2 on this issue: [1], while more accurate references can be found at [2] and [3]).

As of today, NSSDC looks correct. Their Lunokhod 2 page says "The Lunokhod laser retroreflector is still used by Earth-based stations for laser ranging", and their Lunokhod 1 page says nothing. gparker 04:11, 13 July 2006 (UTC)

So what is the precise distance (within experimental error) ?[edit]

The article begins with outlining the (initial) purpose of the LLRE, namely to measure the (average ?) distance to the Moon. But the article doesn't say what this value is ! Surely this value (whatever it is) should be mentioned. MP (talk) 12:00, 27 September 2006 (UTC)


the article says: "The moon is spiraling away from Earth at a rate of 3.8 cm per year, due to the Earth's ocean tides."

I see how tidal friction stopped the moon from rotating, but how can the tides (or continental drift or whatever) affect the moon's orbit? the mass of the earth is always the same (except for meteor dust-- which *increases* it, pulling the moon closer). even if all the water were piled up on the other side of the planet somehow, the center of mass of the earth would be in the exact same place. Sys Hax 10:15, 12 October 2006 (UTC)

Perhaps you should ask on the Science Help Desk, but I think the reason is that the tides cause the Earth's rotation to slow, thus losing angular momentum. The Earth/Moon system has to conserve angular momentum, so the Moon has to move farther away. Bubba73 (talk), 01:19, 30 May 2007 (UTC)

I'm with Sys Hax on this: I think you have to put energy into the earth - moon system in order to get the moon to move further out. I'd expect the moon to be drifting closer as tidal friction bleeds energy from the system. Gbr1918 12:12, 14 September 2007 (UTC)

Think in terms of the conservation of angular momentum, "The conservation of angular momentum in Earth-Moon system results in the transfer of angular momentum from Earth to Moon (due to tidal torque Moon exerts of Earth). This in turn results in the slowing down of the rotation rate of Earth (at about 42 nsec/day), and in gradual increase of the radius of Moon's orbit (at ~4.5 cm/year rate)." Bubba73 (talk), 21:26, 30 November 2007 (UTC)


The article references future improvements in accuracy, and agreement with special relativity to with current accuracy -- does anyone know what current the accuracy of measurement is? —The preceding unsigned comment was added by (talk) 17:27, 16 April 2007 (UTC).

I think it is on the order of 3cm, but don't rely on that. Bubba73 (talk), 01:21, 30 May 2007 (UTC)
There is a paper on ArXiv [4] that explains the three main sources of uncertainty, and the efforts to reduce it. "At present, a single observing session of about an hour yields a distance measurement with uncertainty of about 2 mm". This is not the same as knowing the average orbital distance (center-to-center) to within 2mm. Hadron137 (talk) 04:40, 19 August 2016 (UTC)


SUCK IT, conspiracy theorists! You're wrong. Will you finally admit it now? —Preceding unsigned comment added by (talk) 21:25, 29 February 2008 (UTC)

Yepp , one magic monochromatic photon, doeth not a distance make. — Preceding unsigned comment added by (talk) 12:24, 1 July 2015 (UTC)

So what is the distance to the moon?[edit]

Somebody may want to look into why the distance to the moon stated in this article appears to be wrong. Or at the very least, insufficiently defined as to which measure for the distance to the moon is intended. More commonly, 384,403km is given for the semi-major axis of the orbit and the first reference suggests 385,000km for the mean distance between the centre of the Earth and the centre of the Moon. Neither of these are 384,467km.

Of course prior to its addition on the 2 April 2008, the distance to the moon wasn't mentioned at all in this article which also seems like an odd omission. -- Solipsist (talk) 09:46, 5 August 2008 (UTC)

The mean lunar distance is known to sub-millimeter precision. Can anybody find a reference to a sub-mm 384402XXX.XXXX +/- 0.000Y meter value? Hadron137 (talk) 01:06, 1 March 2016 (UTC)

Aging of the Reflectors?[edit]

Did reflectivity degrade? Because of dirt? Radiation? (talk) 22:00, 17 May 2009 (UTC)Axel

Thermal stresses could be another ploblem.

[5] reports reflectors have degraded to 10% reflectivity and speculates lunar dust may be the reason. (talk) 00:10, 18 May 2009 (UTC)Axel

Ranging experiment being terminated[edit]

It looks like the National Science foundation is terminating funding for the experiments at the McDonald Laser ranging station later this year:

"After 40 years' reflection, laser moon mirror project is axed"

Jnalley (talk) 23:19, 9 July 2009 (UTC)

The experiment will (presumably) continue using the facility at Grasse in the South of Frence. Physchim62 (talk) 08:44, 18 October 2009 (UTC)


I have no courage to edit this page with my bad English, but I do not like this sentence:

one photon of the 10^17 photos aimed at the reflected will be received back on Earth...

I suppose it should be:

one photon of the 10^17 photons aimed at the reflector will be received back on Earth...

Or am I wrong? —Preceding unsigned comment added by (talk) 14:02, 19 March 2010 (UTC)

No you're absolutely correct! A even better way to say it would be "Out of 1017 photons aimed at the reflector, only one will be received back on Earth." Physchim62 (talk) 15:32, 19 March 2010 (UTC)

Speed of light ambiguity[edit]

According to this article,

the speed of light is known with very high accuracy

The phrasing of this is a bit ambiguous; "speed of light" usually refers to c, which 299792458 m⋅s-1 by definition, and so we know its value with infinite accuracy (so it's the length of the metre which we only know to a "very high" degree of accuracy). If "speed of light" refers to c, then I feel the article should be changed to reflect the fact that's precisely defined. Alternatively, if the statement refers to the speed of light in the medium between Earth and the moon, then that should be clarified. I may be a bit of a pedant for feeling that way, but I'd say there's nothing wrong with being pedantic if you're editing an encyclopaedia. Does anyone else agree? (Pre-postscript: if there is a consensus here, please edit it; I'm very likely to have forgotten about this before the day is over.) --Link (tcm) 18:39, 24 June 2010 (UTC)

Simple equation?[edit]

Don't you have to take into account the part of the light path which is within the atmosphere of Earth? —Preceding unsigned comment added by (talk) 16:55, 30 October 2010 (UTC)

Perhaps, but it is a tiny fraction of the total distance to the moon. Jonathunder (talk) 18:32, 28 June 2011 (UTC)
The effects of atmospheric refraction actually produce errors on the order of meters and they must be corrected to get centimeter-level precision. Ryan Hardy (talk) 03:43, 29 June 2011 (UTC)

locations of the reflectors?[edit]

Would anyone be willing to look up the precise locations of the reflectors and add them to the article? — Preceding unsigned comment added by John85 (talkcontribs) 14:43, 21 December 2010 (UTC)

Placement by Apollo 11[edit]

There is some interesting history about how this came about by David Wilkinson . The basics are that something else was supposed to fly on Apollo 11, it couldn't make the flight. NASA engineers were going to put a lead weight in its place. Someone lobbied long and hard to get the reflectors put in its place. The team had to build it on a very short timescale. Ydorb (talk) 02:37, 23 January 2011 (UTC)

New Explanation for Dinosaur Extinction?[edit]

The Lunar Laser Ranging experiment has shown that the moon is moving away from the earth at a rate of about two centimeters a year. If you perform a regression you find that 65 million years ago, the moon must have been orbiting the earth at a height of about 20ft, which, if you think about it, explains why the dinosaurs died out (well, the tall ones anyway). — Preceding unsigned comment added by (talk) 11:30, 7 September 2011 (UTC)

Very funny. (In case you are being serious, try doing the math again. There's a mistake somewhere.) Jonathunder (talk) 15:11, 7 September 2011 (UTC)
Unless there's something wrong with my calculator, 65 million x 2 cm is 130 million cm, which is 1.3 million m, which is 1,300 km, which is about 800 miles, so I don't think even the tallest dinosaurs were in much danger. (talk) 18:45, 4 March 2016 (UTC)
The recent rate of recession (3.8cm/yr) is considered an anomaly. It wasn't always receding so fast.[1] Hadron137 (talk) 01:11, 1 March 2016 (UTC)

EME Bouncing Radiowaves off natural surface of the Moon[edit]

I'm confused about something. If it is a conventional process to bounce radio waves off the natural lunar surface for communications, then why can't this Laser Ranging data be collected by the natural Lunar surface as well? Is there real proof that the lasers are actually hitting man-made retro-reflector on the Moon's surface, and not just designated points on the surface itself? (talk) 15:20, 23 February 2012 (UTC)

First, I'm pretty sure radio signals are not bounced off the Moon for communication - that is why we have communication satellites. Second question: yes, by the strength of the signal bounced back. The retroreflectors are designed to designed to reflect the photons back in the direction from which they came. Even then only a tiny number of the photons are detected returning. They do bounce back from the Moon's surface, but the surface is irregular and they are bounced in all directions, so very few are detected bouncing from the surface itself. Bubba73 You talkin' to me? 16:14, 23 February 2012 (UTC)

Here is the Wikipedia article describing how Earth-Moon-Earth signal bouncing was developed in the 1940's and used for radio communication. (talk) 16:23, 23 February 2012 (UTC)

That (EME) is very interesting, but (1) the retroreflectors are there to measure the distance to the Moon with accuracy in the centimeter range. To do that you need something with a wavelength small compared to a centimeter and the radio wavelengths are much longer than a centimeter. (2) to measure the distance accurately, it needs to have a strong return from a point a precise distance away. Getting returns from the Moon's surface would be all over the place. That is how they can tell that they are coming from retroreflectors - there is a strong signal return from a precise distance, among a weak return from elsewhere. Bubba73 You talkin' to me? 17:47, 23 February 2012 (UTC)
And (3), you need to be able to send and detect individual photons, and I don't think that can be done with radio. Bubba73 You talkin' to me? 17:56, 23 February 2012 (UTC)

Combined Annual Change In Distance[edit]

The measured average annual change in distance was measured between the McDonald Observatory near Fort Davis Texas, and the reflectors placed on the surface of the Moon. The average annual change was actually 38.2 mm per year between the observatory ( Earth Surface ), and the reflectors ( Moon Surface ). What is not widely known is that the annual increase in the elevation of the McDonald Observatory was and is 6.5 mm per year. Combining the two together to get the minimum change in distance between the center of the Earth and the Center of the Moon is thus greater than or equal to 38.2 mm plus 6.5 mm = 44.7 mm per year.

The change in elevation is due to the calibration of the Global Positioning System done in 2003. Originally there were under 1,000 GPS points utilized to calibrate the X, Y, and Z co-ordinates of the Global Positioning System. Of that number of points about 10% were decreasing in elevation excessively, 10% were increasing in elevation excessively, and the rest plotted as a nearly straight line of annual delta Z's between minus 8 mm per year to plus 16 mm per year. the mid-point showed a delta Z of plus 4.15 mm per year, and the average of 720 points was plus 4.17 mm per year. McDonald Observatory was one of the 720 calibration points at plus 6.5 mm per year. In short is showed that the Earth ( mostly land ) was increasing in elevation relative to it Center between 1965 and 2003. See WGS - 84 ( World Geodetic Survey 1984 ).

The original data included more than 900 points when it was originally posted on the web, but recently I looked at the same data, and now it includes only 157 points. The only data now shown fall between minus 4.0 mm and plus 4.0 mm with an average change of zero mm. Thus the elevation of the land ( magically ) no longer increases or decreases in elevation for the Global Positioning System. This may be good for the Global Positioning System, since it is used for surveying, but it is regrettable when the data are needed for other things like determining the true change in distance between the centers of the Earth and Moon.

Cepheid Variables ( spinning and vibrating stars ) and radio telescopes were used to make the calibrations along with Satellite Laser Ranging to find the Delta X and Delta Y ( continental motions - drift ), and also the Delta Z ( vertical motions ). (talk) 00:10, 19 January 2013 (UTC)

Expansion of the Universe[edit]

Having read on the Wikipedia page that the Universe expands at the rate of 67 km per second for every million parsecs of distance, I applied this ratio to the Moon Earth distance and found that this would increase the distance between these two bodies by about 2.6 cm per year, (if my ratio calculations are correct). Could this explain why the values for the transfer of angular momentum seem too high. When measuring this distance there are two effects being seen and that we are observing the expansion of the Universe here in our own solar system! — Preceding unsigned comment added by AgeofApollo (talkcontribs) 15:17, 11 April 2014 (UTC)

You calculation is not right: 67 km * ((384.40000 * km) / (1 Parsec * (10^6))) * 60 * 60 * 24 * 365 = 0.00263217187 centimeters -- (talk) 21:57, 23 September 2014 (UTC)

Wikipedia:xkcd in popular culture[edit]

— Preceding unsigned comment added by JoelDick (talkcontribs) 03:12, 19 November 2014 (UTC)

I think what you're trying to say is that this article could use an 'In popular culture' section that mentions this xkcd page? -- (talk) 23:06, 7 January 2015 (UTC)
Anyway, before you added your comment here, someone had already added it and someone else already removed it because of WP:XKCD. You know, it would really help if, in the future, you don't just link to an essay, but add a couple of words that explain why you linkdump something. -- (talk) 23:18, 7 January 2015 (UTC)

What is the return dispersion?[edit]

When the laser arrives at the moon it is 6.5km wide (due to early atmospheric dispersion etc?). How wide is the beam when it gets back to earth? I'm not sure what to expect... it could be less due to the long straight travel through space, or more if the laser optics are straighter than the angle of reflection at 384000km.

Is the light more concentrated in the middle when it arrives back at earth? What effect does the earth's rotation (of about 1.25km) have on the strength of the received signal? Skestle (talk) 22:54, 16 June 2016 (UTC)

  1. ^ Walker, James C. G.; Zahnle, Kevin J. (17 April 1986). "Lunar nodal tide and distance to the Moon during the Precambrian". Nature. 320 (6063): 600–602. doi:10.1038/320600a0.