Lunar Laser Ranging

[pbethala]

Hi, I have jsut read an article about lunar laser ranging to try to measure the moon's exact position to confirm Einstein's General Theory of Relativity. I do have a question, though. Why can't the receivers simply detect photons from the moon to determine its position rather than use a laser, which results in a far less number of photons for measuring? I know this is a stupid question, but I am hoping for an intelligent answer.

[Kaptain K]

You can't measure the distance by using ambient light because you have no idea when it left the Moon.

By using a laser, you know the time the beam left the Earth, to less than a nanosecond and the time it returns, also within a nanosecond. Therefore you know the round trip time.

Knowing the speed of light, it is easy to calculate the distance.

[pbethala]

But isn't the speed of light constant? That is, if we see light from the moon, we would see it as it was about a nanosecond ago (according to the measurements), just like the laser, only sooner. Right? (I doubt it)

[01101001]

But isn't the speed of light constant? That is, if we see light from the moon, we would see it as it was about a nanosecond ago (according to the measurements), just like the laser, only sooner. Right? (I doubt it)

(It takes about 1.28 seconds, not a nanosecond. It's a long ways to the moon.)

I don't understand how you would measure the distance using light from the sun. Say you happen to measure a photon to arrive from the moon exactly at midnight. When did it leave the moon?

[EndeavorRX7]

Good question. We could use the light reflected from the moon to measure distance. However, most light reaching earth from moon is defrated. In terms of nanoseconds it is much more accurate to measure the roundtrip of a laser beam in we control it's departure.

[Kaptain K]

What does defrated mean?

[pbethala]

(It takes about 1.28 seconds, not a nanosecond. It's a long ways to the moon.)

I don't understand how you would measure the distance using light from the sun. Say you happen to measure a photon to arrive from the moon exactly at midnight. When did it leave the moon?

About 1.28 s earlier? Sorry, but I still don't seem to be grasping the concept.

Good question. We could use the light reflected from the moon to measure distance. However, most light reaching earth from moon is defrated. In terms of nanoseconds it is much more accurate to measure the roundtrip of a laser beam in we control it's departure.

This explanation seems to make sense to me. Thanks.

[jwheel]

See the following link for more info about Lunar Laser Ranging:

http://www.csr.utexas.edu/mlrs/

[Kaptain K]

About 1.28 s earlier? Sorry, but I still don't seem to be grasping the concept.

We know that it takes about (operative word) 1.28 seconds for light to get here from the Moon because we know the speed of light and we have a pretty good idea of how far away the Moon is from other means (parallax). We have no means of knowing when any given photon from the Sun reflected off the Moon (they don't have serial numbers!). Using lasers gives us a way to tell photons apart! We know exactly when this photon left Earth and exactly when it got back. Therefore, since we know its speed, we know exactly how far it traveled.

[Kaptain K]

Look at it another way.

You are standing at the edge of "echo canyon" and your twin is on the other side.

Your twin yells and you hear him. How far away is he? You have no idea, because you don't know when he yelled!

Now, you yell and time how long it takes to hear the echo. Since you know the speed of sound, it is easy to calculate the distance across the canyon.