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Science

Measuring The Distance From Earth To Moon 284

Posted by timothy from the technical-challenges dept.
lewiz writes: "Tom Murphy at UW is attempting to measure the distance between the Earth and The Moon to the nearest millimetre according to this BBC News article. 'His tape measure will be a giant telescope at Apache Point in New Mexico. Retroreflectors left on the surface of the Moon by various space missions, including the Apollo 11 lunar landing, will also come in handy.'"
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Measuring The Distance From Earth To Moon More

Measuring The Distance From Earth To Moon

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  • ...if the moon landing *actually* happened.
  • Isn't this better than moving closer to Earth?

  • Oddly Enough... (Score:4, Interesting)

    by CheshireCatCO ( 185193 ) on Wednesday January 16, 2002 @12:43AM (#2846767) Homepage
    ... according to my Solar Systems Dynamics textbook, we already know the rate at which the Moon is receding from the Earth: 1 nanometer/second. Which is, of course, a better precision than this group seeks to take on. But that measurement probably used the Doppler shift. It just goes to show that it's much easier to measure radial velocities than distances!
    • shhh!!!! don't tell the organization that's funding their research that!!!!

    • Re:Oddly Enough... (Score:5, Informative)

      by p3d0 ( 42270 ) on Wednesday January 16, 2002 @12:57AM (#2846827)
      Bzzztt. The speed that the moon receeds wasn't measured directly, especially by doppler shift of all things. It was calculated from the kinetic energy gained by the moon from the Earth from tidal forces.
    • I hate to second guess people (OK, makbe I don't, so sue me), but I just hope they do take this movement into account when they do their measurements, since it will take time to bounce signals back and forth, and more importantly nanometers will add up over several years until they are just a little bit significant. (60 sec x 60 min x 24 hrs x 365 days x 5 yrs x 1 nanometer = 157,680,000 nanometers, or .15768 meters) Feel free to check my math, it's late...

      • Re:Factor that in (Score:2, Insightful)

        by oo7tushar ( 311912 )
        Not exactly sure what you mean by "since it will take time to bounce signals back and forth".

        We know that the moon recedes at 1 nanometer per second. Since they're trying to determine nearest millimeter they don't care about how much it's receding or even if it is. As far as they're concerned, it doesn't move farther away in a day or even a few months.

        Also, it takes just over 2 seconds for light to get there and back so we're looking at a 2 nanometer change in the whole time. Also, this nanometer movement will change as the moon gets farther away.

        Thus they don't have to worry about movement (tis less than the 5% error range they're "allowed")

        hope that's kinda helpful

    • Re:Oddly Enough... (Score:1, Interesting)

      by Anonymous Coward
      1 nanometer/second. Which is, of course, a better precision than this group seeks to take on.

      Hmm, one significant digit isn't very precise. And significant digits are all that matters: any measurement in centimeter/year can be converted to nanometer/second (or to attometer/nanosecond, or to meter/century...) it's just a speed that is measured.
    • according to my Solar Systems Dynamics textbook, we already know the rate at which the Moon is receding from the Earth: 1 nanometer/second. Which is, of course, a better precision than this group seeks to take on.
      At the bottom of the BBC article is a link to a previous BBC article [bbc.co.uk] from 1999 that says that the "McDonald Observatory Laser Ranging Station near Fort Davis in Texas, US, regularly sends a laser beam through an optical telescope to try to hit one of the reflectors". So I'm not sure what's special about the new article. Anyway the McDonald Observatory people have measured the velocity of the moon's recession at about 3.8 centimetres per year. That's where the 1 nanometer per second figure comes from. So it's not better precision at all, they've just used very small units.
  • "This is essentially measuring the weight of gravity, and this is the only type of project that can currently do that[...]"

    Huh? since when did forces have weight
    • Force = Weight.

      the formula for Force is: F=ma, replace a with the acceleration caused by gravity and you got weight.
      • F != W.

        Weight is the measure of attraction between two masses, such as the gravitational force between the Earth and you. But force certainly does not equal weight.

        Force is also a vector, which weight is not.

        • Weight is too a vector. You weight X lbs, straight down. :)
          • Weight is a gravitational force, thus acting on 2 objects, and pulling them either towards or away from each other. I don't know of any bi-directional vectors.
        • Weight _is_ a vector. Weight _is_ a type of
          force. Remember, mass is a number, weight is
          a vector.
          I think when they say weight of gravity they
          mean that gravitons or grabity waves have
          non-linear dynamics, i.e. they interact with
          themselves and Einstein equations are needed to
          deal with this self-consistently. But the simplest
          way to correct Newtonian gravity is to analyze
          corrections from gravity interacting with itself,
          which could be worded as measuring weight of
          gravity.
          • Weight _is_ a vector

            Well, yes and no. I know the equation suggests it is (W = m.g, where bold represents vectors. But weight is not used as a vector, and it leads to silly conclusions if it is.

            For example, I weigh myself here to be 100kg (or 980N) - what is the direction of this weight vector: towards the centre of the earth. Thought experiment time. Two 100kg men weight themselves, on at the north pole, one at the south pole. What is the sum of their weights?

            Well, if weight is a vector then the sum is zero. If, however, you take weight as a scalar then it is 200kg. What people mean when adding weights only works if weight is a scalar. Basically, defining weight as a vector fails the common sense, similar to defining glass as a liquid as opposed to a solid.

            All that is happening is that specialists are taking a word that is in common usage (weight in this case) and defining a new (and different) techincal meaning for the word, which is similar to but distinct from the everyday meaning. Then they complain when people use it in the everyday sense.
            • You are right, but you forget one thing. The specialists are the one using the word consistently, if differently from everyone else. It is the nonspecialists who sometimes use it to mean mass, and sometimes use it to mean the vector. It's pretty obvious which party is confused, and which party isn't.

  • I mean, it's all well and good to say you're going to measure this sort of thing, but how can we be sure he's correct? I could just as easily say that the exact distance from the center of the earth to the center of the moon is 385,137.473 KM.

    In fact, what if these scientists are just using this as a cover to get a lot of funding money. Look for the research team to be on vacation in the Bahamas next month. When they come back, they've got a request for funding on a project about angels and pinheads...

    • Same way we ever check, have another group do an indepenent measurment or make them show their data and analysis?

      • Re:How do you check the accuracy? (Score:2, Insightful)

        by Anonymous Coward
        Umm, did you miss the point? first off, how do we know the second team didn't just take the money, guesstimate and take a trip to the islands? Second off, if the second team is wrong, who's right? You want to bring on a third team? Point is, in Science you never know if you're right until your observations match up to a well formed theory, within a margin of error. There is no theory which dictates the distance from earth to moon on the millimeter scale, therefore there is no check on our observations. There is no way to tell that some whacked out relativistic effect is affecting our measurement, or that the methodology is just plain wrong and nobody has noticed yet. Look into Thomson's original measure of the charge of a single electron. His experiment was extremely clever, but in the calculations he forgot to take into account the viscosity of air. As time went on, various experimenters reproduced the experiment, but somehow their data was just a little higher than Thomson's, so they chalked it up to bad data, phase of the moon, god playing dice, etc. Nobody dared doubt that the great Thomson's clever experiment could be wrong.
        • There is no theory which dictates the distance from earth to moon on the millimeter scale, therefore there is no check on our observations.

          Yes there is. Newton's laws, modified by relativity, and a big of geometry allow you to calculate the distance using only the mass of the earth and the orbital period. In theory, if you had the mass & the orbital period known to enough accuracy, then you could calculate the distance.

          The newtonian formulas are:
          a=v^2/r, a=acceleration, v=orbital velocity, r=radius.
          a = G m / r^2, G=Newton's constant, m = mass
          v=circumfrence / orbital period

          With known m & orbital period then you can solve for r.

          Of course, in real life, it's going to be much easier to measure the distance to the moon than the mass of the Earth, and the theory ignores the influence of the rest of the universe, so it's not going to be 100% accurate, so even in Newton's time they measured it, except they used parallax methods.

  • Umm, the distance isn't constant (Score:4, Interesting)

    by DeafDumbBlind ( 264205 ) on Wednesday January 16, 2002 @12:48AM (#2846789)
    The moon revolves around the earth in an eliptical orbit, not a circular one. Unless he's talking about getting the average distance.

  • 3.8 cm (Score:1, Interesting)

    by niola ( 74324 )
    Well, if the moon is moving 3.8 cm away from the Earth every year, that does not bode well for life on this planet next millennium. Since gravity is so important to our climate I would imagine that this could have bad long term effects.

    I remember reading something a while back about ancient Egyptians believeing there were two suns int he sky. I wonder if what they saw was a much closer moon back then?

    All throughout the Earth there is evidence of dramatic gravitational changes int he past. For example, below all the ice in the northern reaches of Canada there are vast reserves of coal. Since coal is made through the decay of organic matter, it means there had to be a massive forest up there at some time. Then in the deserts of Africa there are mountains where they find seashells like 1000 feet above sea level.

    Perhaps the moon pulling away from the Earth has caused many of the major changes on the Earth...

    --Jon

    • Re:3.8 cm (Score:2, Insightful)

      by Hitokage_Nishino ( 182038 )
      Think about what you are saying.

      The moon is currently around 260,000 km at its nearest point in orbit. 3.8cm per year is not a significant distance over a couple years, or even 10,000 years(by which the moon has drifted 380m).
      • Not sure why the parent was marked as a troll, but given the age of the Earth, and a movement of 380 m in 10,000 years, I would assume it could have had a significant effect on the Earth millions of years ago.
        • At the rate of .0038 m/yr, a billion years ago it was only 3800 km closer. Out of roughly 300,000 km between us and the moon, it still doesn't make much difference.
          • And that's assuming that the rate is constant. In actual fact it's not, it's increased over time.
    • 3.8cm/year * 4000 years = ~ 150m

      Not far enough to differe
    • As others have pointed out, 10,000 years at 3.8 cm per year is only 380 meters. Out of over 250 thousand kilometers, that's not much change.

      The gravitational effects of the moon on the Earth are fairly minor. Losing the moon would kill the tides, and creatures that depend on them, but that's about it.

      The standard explanation for coal in cold areas and seashells way above sea level is continental drift and tectonic action. No moon gravity explanation necessary.

      • Re:3.8 cm (Score:2, Informative)

        by lewellyn ( 38689 )
        For a much more rational and thought out theory on the origin of "fossil" fuels and other hydrocarbons see Thomas Gold's website [cornell.edu].

        His book, The Deep Hot Biosphere is very insightful and most of the information and theories in it are also covered in depth on his website.
    • All throughout the Earth there is evidence of dramatic gravitational changes int he past. For example, below all the ice in the northern reaches of Canada there are vast reserves of coal. Since coal is made through the decay of organic matter, it means there had to be a massive forest up there at some time. Then in the deserts of Africa there are mountains where they find seashells like 1000 feet above sea level.

      Perhaps the moon pulling away from the Earth has caused many of the major changes on the Earth..

      *cough*Plate Tectonics!*cough*

      Coal was generally formed from about 300 to 65 million years ago. That's a lot of time for them continents to wander. That 'massive forest up there' was probably some massive forest down here 300 million years ago.

    • Re:3.8 cm (Score:2, Informative)

      by dragons_flight ( 515217 )
      Once upon a time, the moon was part of the Earth. Billions of year ago, well before any known evidence of life on Earth, the partially formed Earth was hit by a planetesimal in the primordial solar system. The impact caused a large glob of primarily molten material to spew off the surface of the Earth and coalesce into what is now the moon.

      Ever since then the moon has been slowly drifting away from the Earth. It doesn't drift away because of the original impact, that energy long since dissipated; the moon is sliding back as tidal forces between the Earth and the moon dissipate small amounts of angular momentum. Eventually the Earth and the moon will become face locked, so that not only do we see the same side of the moon all the time, but the Earth will have slowed so that the same side is always facing the moon. This is a long time away since the earth day will have to slow till it's as long as the moon's period or about 30 times longer than today's day. The moon will not escape however.

      To answer your question though the moon has moved less than 1/10000 of a percent since the Egyptians, so no it wouldn't seem that much bigger. In the last 50 million years the moon would have moved around 1%. Because the lunar interactions slow the Earth's spin we know that the Earth probably once spun around twice as fast as it does now (12 hour days). Yes the changes would have had an impact on the Earth and on life, but the change is very very gradual. If you want to look for big effects on the Earth you probably ought to consider more drastic influences like volcanism, earthquakes and large meteors. And just to confuse it all, plate tectonics has no end of fun moving stuff around on the surface of the Earth.

  • Proof of Moon mission? (Score:1, Insightful)

    by Anonymous Coward
    So if they are using stuff that the Apollo missions supposedly left up there, will that prove once and for all to the conspiracy theorists that we actually did go to the moon?

  • Alternative Solution (Score:5, Funny)

    by nurightshu ( 517038 ) <rightshu@cox.net> on Wednesday January 16, 2002 @12:50AM (#2846799) Homepage Journal
    ...Check the Odometer on the Apollo 11 capsule.
  • I don't think the earth has a perfect orbit with the moon, so I would assume that the distance is not entirely constant. Am I wrong? Can anyone shed some "light" on the subject?

  • But my old encylopedia... (Score:4, Funny)

    by heliocentric ( 74613 ) on Wednesday January 16, 2002 @12:53AM (#2846807) Homepage Journal
    My old encylopedia says it's precisely 300,000 km and it also says that "someday we hope to go there."

    All that and the Kaiser Wilhelm still doesn't know what to do with his country.
  • Gee, if they have a telescope that can see stuff on the moon in that kind of detail, does that mean that they can now confirm that the moon landing wasn't faked?

    Cryptnotic
    • Gee, if they have a telescope that can see stuff on the moon in that kind of detail, does that mean that they can now confirm that the moon landing wasn't faked?
      Uh... what? Nobody's talking about about looking at the Moon at a high resolution - merely measuring the distance from the Earth to the Moon with high precision. You don't need to see the mirrors the Apollo landers left to use them. The beam spot size of a laser projected from Earth is a few kilometers wide when it gets to the Moon.
    • Ha, you're going to 'confirm' this supposed moon landing by showing doctored photographs ? Try again.
  • It's been a long time there are permanent earth stations around the globe doing that. Maybe bigger telescope is more precision (I've heard of centimeter-precision so far).

    It was the theme of a science exam I took in 1994 to enter a college. Every aspect of it, including the ways the moon mirrors are designed so that they always send the light back to its source (vaguely mentionned in the article)

  • Wipe-cut to the next scene... (Score:4, Funny)

    by Tony Shepps ( 333 ) on Wednesday January 16, 2002 @01:04AM (#2846860)
    ...his wife [sobbing]: "Dammit Tom! You could measure how far it was to the moon! But you couldn't see the distance between... between US!

  • Useful? (Score:3, Funny)

    by Nick Smith ( 321087 ) <nsmith.webone@com@au> on Wednesday January 16, 2002 @01:09AM (#2846878) Homepage
    Why can't he do something more useful? Like measure the cheese composition of the moon...?

    Current estimates predict that all known gorgonzola deposits on Earth will be depleted by 2016. We need to think about cheese-mines on the moon now, before it's too late...

  • Mooting points. (Score:5, Informative)

    by blair1q ( 305137 ) on Wednesday January 16, 2002 @01:10AM (#2846882) Journal
    - The moon's orbit about the Earth is a 100-page equation, not a constant.

    - The Earth's rotation is not circular (it "sloshes").

    - The Earth's shape is not constant.

    - The Earth's mass is not constant, so the general relativistic field in which the moon orbits is not constant.

    - Okay, so we know where that telescope is relative to the moon. Now where is it relative to my house? To Washington? To Wendy's?

    - Isn't this just an attempt by the Bush White House to wag the dog to distract attention from the fistfight the President and Vice President had during the game Sunday?

    --Blair
  • Are those anything like, um, reflectors? Or mirrors, even?

    • Re:'retroreflectors'? (Score:2, Informative)

      by zudark ( 11987 )
      Like them, yeah... :) But not just mirrors. Retroreflectors reflect light back on a course essentially parallel to the incoming rays. This makes them kind of creepy to look into, since no matter how you rotate them, there's an eye staring DIRECTLY back at you :)

      To see this in person, walk up to a surveyor at a construction site sometime... if you're lucky and they're using an optical total station, there'll be someone walking round with a pole w/ a corner cube reflector (a type of retroreflector) on top. This is used so that the pulses of light coming out of the total station get reflected back to the station no matter how the pole guy has the pole oriented.

      Another good example is the material highway signs and license plates are coated with -- they show up in your headlight beams so well because much more of the light reflecting from their surface heads back toward the light source (the headlights, very close to being in line with your eyes) rather than being scattered or reflected off into a less useful direction.

      Check out http://www.leica-geosystems.com/ims/product/tps500 0_reflectors.htm for some pics...
      • Another common use is the material that they put on kids & workout clothing so that they show up well at night. The light from the car headlamps hits the material, is reflected back parallel to the original beam, and thus the driver can see the kiddies before he hits them.

  • Reflectors on the moon? It's a lie !!! (Score:5, Funny)

    by bani ( 467531 ) on Wednesday January 16, 2002 @01:16AM (#2846902)
    ... according to Bart Sibrel [moonmovie.com] :D

    To sum up: "we never went to the moon, hence there are no reflectors on the moon."

    Moderators: Put down the crack pipe and the mouse. Step away from the keyboard. Take a DEEP BREATH and READ. This is not a troll. I repeat. This is not a troll.

    No, I don't believe the "moon hoax" loonies. Anyone with an above-kindergarten education can easily refute the "moon hoax" loonies claims.

    For your amusement, this is the gallery of the 'barking mad'...

    Some anonymous kook [geocities.com]
    Bill Kaysing [okneoac.com]
    Ken Overstreet [8m.com]
    "mpeeters" [geocities.com]
    • I hate to ask... but, has anyone actually seen the video on moonmovie.com? I am wondering if it is any good or not.

      another question: don't we have telescopes good enough that we could see things that were "left" on the moon, like the lunar rovers, their boots and gloves, etc.? It would seem that would put the "did we really land on the moon?" controversey to rest....

      • I don't have a link, but I recall reading ( I think it was a FAQ from HBO's Earth to moon series) that the atmosphere limits image resolution to a point that it isn't possible to get an image from a land based telescope that will show the decent modules and lunar rovers left on the moon.

        I also suspect that orbiting telescopes can't be configured to focus so close. ( like trying to use binoculars to look at your hand)

        It does leave me wounding just how powerful the laser must be to be scattered by the atmosphere on the way up and the way back and still be visible.
    • For your amusement, this is the gallery of the 'barking mad'...

      The definitive list can be found on crank.net [crank.net]. If you ever want a good laugh, try reading through the sites listed as "illucid". ;-)

      Why do cranks always pick on either cosmology or the moon? I want to see a crank theory of horizontal branch morphology or AGB stars! :-)=

      [TMB]

  • From the article:
    But the task is not as simple as it sounds. The beam of light must hit the retroreflectors, each about the size of a suitcase, on the lunar surface.

    This is made even trickier by the fact that the beam will be about 2 kilometres (1.2 miles) wide by the time it reaches the Moon.

    Stupid gits!

    Much easier to hit that suitcase-size reflector with a beam that's more than a mile wide than with one that's pencil thin!

  • How refreshing to know our tax dollars are going to be spent to fund this project for five years.

    This is like trying to measure Bill Gates' wealth to the penny.

    The moon's orbit varies by far more than 1 millimeter all the time. There are all sorts of influences, including the earth's gravity, the sun's gravity, Jupiter's gravity -- as well as factors that will introduce error into the measurements, such as refraction of the laser in the earth's atmosphere.

    Besides, I've already measured the distance with my trusty golf rangefinder. :o)

    -- CP
    • The moon's orbit varies by far more than 1 millimeter all the time. There are all sorts of influences, including the earth's gravity, the sun's gravity, Jupiter's gravity -- as well as factors that will introduce error into the measurements, such as refraction of the laser in the earth's atmosphere.

      Therefore, by measuring the distance to an accuracy of a millemeter, we might be able to gain some insight about: earth's gravity, the sun's gravity, and Jupiter's gravity. The point of this experiment is not to know how far it is to the moon. The point is to better understand the moon's orbit, and the various influences which affect it.

  • Parallel space.com article (Score:1, Funny)

    by Anonymous Coward
    The parallel article [space.com] contains this ghastly analogy:

    Murphy's real motivation is to test Einstein's General Theory of Relativity, which is
    based on an assumption that gravity affects a feather and a bowling ball in the same
    manner. You cannot test this in your home, because air keeps the feather aloft.

    I forget -- was Einstein the guy who dropped the balls off the tower or the one who was hit in the head with an apple?

  • Which part of the Earth? (Score:3, Insightful)

    by guttentag ( 313541 ) on Wednesday January 16, 2002 @01:54AM (#2846987) Journal
    The article says we already know the distance between the center of the Earth and the Moon, but Murphy wants to get as accurate a measurement as possible, and suggests that it will be accurate to the nearest millimeter.

    Unfortunately, he's not really measuring the distance between the centers of the two planetoids... he's measuring the distance between the top of some piece of equipment on the Moon and the lens of his telescope.

    This reminds me of the human genome project. At some point, the scientists announced, "we've finished mapping the human genome! It's finished!" And as it turned out, it wasn't anywhere near completion. I believe it's still unfinished.

    Scientist: "We're going to measure the exact distance between the center of the earth and the center of the moon."
    Janitor: "So, why is it off by billions of millimeters?"
    Scientist: "Picky, aren't we? It's close enough!"

    Is modern science really so desperate to inspire interest in people? Lies are not impressive.

    • Right. I'm sure they won't take into account the fact that the telescope isn't at the center of the earth.

      I believe these people know more about what they are doing than you do. At least give them the benefit of the doubt. But wait... you don't even have a reason to doubt.

      Well, failing that, don't talk unless you know what the hell you're talking about.

  • Oh no! (Score:2, Insightful)

    by ImaLamer ( 260199 )
    Tom: Umm... the text book has 385,137.473 KM

    Fellow worker: So?

    Tom: I've got 385,137.471, should warn someone?.... ah, um... what are you doing?

    Fellow worker: CALLING THE WHITE HOUSE!

    What should we expect from this 'experiment'? A warning that the moon is going to crash into the Earth in 2003? Come on! Put the money towards something useful, like... figuring out why we aren't on Mars when NASA admits we could have been there in '85 if we kept on schedule.
  • $ ping moon
    PING moon (212.58.226.40): 56 octets data
    64 octets from 212.58.226.40: icmp_seq=0 ttl=2000 time=1.283 s

    1.283 x 300000000 = 385000000

    So the moon is 385000 km from earth. Easy!

  • Another totally worthless experiments (Score:4, Interesting)

    by trenton ( 53581 ) <trentonl@NOSPAm.gmail.com> on Wednesday January 16, 2002 @04:23AM (#2847208) Homepage
    How about this one: build a clock that can be taken to sea. What good is that? We already have accurate clock on the shore. Oh, right, you can figure out your latitude [nmm.ac.uk]. And, in the process, invent roller bearings and bimetallic strips [nmm.ac.uk], two extremely useful technologies, ones we weren't even trying to create. That turned out well for us, huh?

    The moon distance measurement is obviously good. It seeks to do something no one else has ever achived. Even if the results aren't interesting, the new techniques used are. Anyone that doesn't see its intrinsic value is shortsighted.

  • "Ok, I'll hold THIS end..."
  • Lots of people seem to know that the moon is moving farther away from the earth, but did you know that the earth's rotation is slowing down?

    These two phenomena are actually related -- the orbital angular momentum that the moon is gaining (moving farther away) is taken from the earth's rotational angular momentum. Gradually, but measurably, the earth *is* slowing down. You might have heard of leap seconds? These are to compensate (partially) for the slowing in fact!

    Eventually (ok, in maybe 100 million - 1 billion years) the earth and moon will orbit/rotate at the same angular velocity, so that at that time, we will always see the same face of the moon, and the moon will always see the same face of the earth!

    Better pick which side of the earth you'll want to live on, otherwise if you pick wrong, you'll *never* be able to see the moon... :)

  • If all of the articles published on this topic were stacked one on top of the other, they would reach the moon and back 3.42 times.

    Or should that be 3.46?

  • significant digits / error / order of magnitude (Score:3, Insightful)

    by SCHecklerX ( 229973 ) <greg@gksnetworks.com> on Wednesday January 16, 2002 @10:09AM (#2847788) Homepage
    "Tom Murphy at UW is attempting to measure the distance between the Earth and The Moon to the nearest millimetre

    That's pretty stupid, considering the distance will obviously change more than a millimeter all the time. Hell even the astronaut's footprints will be several millimeters thick, assuming they are undisturbed still. So...within a millimeter of *what*, exactly?

  • How do you get an "accurate to one millimeter" measurement to an object covered with dust grains, plus pebbles, rocks, boulders, mountains and craters? Doesn't your value change depending on whether you measure to the top of that 1 mm sand particle. or to the rock it's on?

    OK, they say they are trying to measure the center to center distance, but they don't get that directly. The real measurement is from a telescope mount on top of a mountain on Earth, to a retro-reflector on the Moon. Do you actually know the height of that mountain at the observatory to 1 mm? And can you correct that height to the day the measurement was taken? (Some sorts of subsoil will shrink and swell depending on water contact, sometimes resulting in the ground rising and falling a few feet annually. I'd think that deep down in a mountain would be rock so it wouldn't do that, but in most cases the whole mountain is either rising or falling by at least millimeters a year, and if there is any soil cover weather changes might change the height by a few millimeters.)

    And on the moon, you are measuring to a reflector which is basically laying where the astronauts dropped it 30 years ago. How would the distance from the reflector to the center of the moon be measured? Laser beams & navigational gear in satellites orbiting the moon? What satellites?
  • "But the task is not as simple as it sounds. The beam of light must hit the retroreflectors, each about the size of a suitcase, on the lunar surface.

    This is made even trickier by the fact that the beam will be about 2 kilometres (1.2 miles) wide by the time it reaches the Moon."/I>

    Oh-kay.... so it would be less tricky if your laser beam was, say, 2 millimeters wide by the time it reaches the moon?

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