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

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: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.

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?