Measuring The Distance From Earth To Moon

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.'"

Comments

How do you check the accuracy?

[nurightshu]

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...

Re:How do you check the accuracy?

[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.

Proof of Moon mission?

[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?

Re:Am I reading this right?

[Peyna]

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.

Re:3.8 cm

[Hitokage_Nishino]

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).

Re:Am I reading this right?

[Compuser]

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.

Which part of the Earth?

[guttentag]

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.

Oh no!

[ImaLamer]

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.

Re:Factor that in

[oo7tushar]

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

Improved correction

[Anonymous Coward]

One measurement as of 1994 was: 3.82±0.07 cm/year

The reference is: Dickey, J.O. et al.,
"Lunar laser Ranging: A Continuing Legacy of the Apollo Program"
Science 265: 482-490. (July 22, 1994)

(And just to be absolutely clear, I am not suggesting that they used the Doppler effect! They used the same direct measurement technique described in the BBC article referenced by slashdot.)

Re:Am I reading this right?

[pmc]

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.

significant digits / error / order of magnitude

[SCHecklerX]

"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?