Moon Mirror

[Johnny Pneumatic]

Easy proof that we went to the moon. this (http://www1.nasa.gov/vision/space/features/21jul_llr.html)

[LostAngeles]

Wow.

Now that is badass. Why hadn't I ever heard of that before?

[pgwenthold]

Originally posted by bewareofdogmas
Easy proof that we went to the moon. this (http://www1.nasa.gov/vision/space/features/21jul_llr.html)

In terms of the distance measurements changing, have they accounted for the change in the speed of light?

[ducking]

[Johnny Pneumatic]

Originally posted by pgwenthold
In terms of the distance measurements changing, have they accounted for the change in the speed of light?

I thought that was shown to be incorrect.

[Brown]

Originally posted by bewareofdogmas
I thought that was shown to be incorrect. I think that pgwenthold's remark may have been meant in jest, but the speed of light does indeed change. Light travels just a bit faster in a vacuum than it does in air. (If you remember your high school physics, the speed of light through a medium is related to its index of refraction.) It would not be unreasonable to ask whether the distance calculations take into account the fact that the light makes part of its round trip through atmosphere.

[pgwenthold]

Originally posted by Brown
I think that pgwenthold's remark may have been meant in jest, but the speed of light does indeed change. Light travels just a bit faster in a vacuum than it does in air. (If you remember your high school physics, the speed of light through a medium is related to its index of refraction.) It would not be unreasonable to ask whether the distance calculations take into account the fact that the light makes part of its round trip through atmosphere.

However, since the path through the atmosphere is pretty much the same from year to year, such an effect would not make it appear the moon is moving.

[Brown]

Originally posted by pgwenthold
However, since the path through the atmosphere is pretty much the same from year to year, such an effect would not make it appear the moon is moving. That may be right. I think the speed of light through the atmosphere might vary depending on heat or cold or fumes in the air, but basically I suppose the atmosphere is pretty constant. So the fact that the average distance from the Moon gets bigger every year would not be an atmospheric phenomenon.

I would suspect, however, that atmosphere might account in part for some of the imprecision in individual distance measurements. But then again, maybe not. If I remember correctly, the speed of light in air is very, very close to the speed of light in a vacuum, so maybe the difference in speed is trivial for purposes of these measurements.

The error of individual measurements is, according to the story, "a few centimeters." And the Moon moves a few centimeters further away (about 3.8 cm) every year. (Perhaps this refers to the semi-major axis increasing by about 3.8 cm per year? During any given month, the Moon moves toward the Earth and away from it.) Chances are that it takes a slew of measurements to show this effect, due to the inherent imprecision of the measuring technique.

[neutrino_cannon]

Alternatively, I had heard that lasers were useful to astronomers for clearing atmospheric distortion, so perhaps the first part of the pulse deals greatly with atmosperic distortion.

[tracer]

Originally posted by neutrino_cannon
Alternatively, I had heard that lasers were useful to astronomers for clearing atmospheric distortion,
My understanding was that the lasers in the super-fine-resolution Earth-based telescopes don't CLEAR atmospheric distortion, they MEASURE it so that computers on the ground can compensate for it by moving mirrors in the reflector array or by subtracting out the distortion effects via image processing.

[neutrino_cannon]

Originally posted by tracer
My understanding was that the lasers in the super-fine-resolution Earth-based telescopes don't CLEAR atmospheric distortion, they MEASURE it so that computers on the ground can compensate for it by moving mirrors in the reflector array or by subtracting out the distortion effects via image processing.



never mind

[Matabiri]

Originally posted by bewareofdogmas
I thought that was shown to be incorrect.

Still undecided, I think. Some theories (sorry, hypotheses) allow it to - or at least, allow the Fine Structure Constant to change; this is usually interpreted as a change in the speed of light.

Measurements for this are often done using the natural fission reactor at Oklo (http://www.physics.isu.edu/radinf/oklo.htm), in Gabon. According to an article in New Scientist (http://www.newscientist.com/news/news.jsp?id=ns99996092) reviewing this recently, people keep thinking they find evidence, then someone finds evidence that it hasn't changed (in 2 billion years, at least), then someone'll find new evidence... etc. etc.

[wollery]

Originally posted by tracer
My understanding was that the lasers in the super-fine-resolution Earth-based telescopes don't CLEAR atmospheric distortion, they MEASURE it so that computers on the ground can compensate for it by moving mirrors in the reflector array or by subtracting out the distortion effects via image processing. [slight derail]It can be done with lasers, but it's usually done by measuring an off axis star and, assuming it should have a circular profile, distorting the mirrors to compensate. It's called adaptive optics, and if you want to know more I suggest this page (http://www.eso.org/projects/aot/introduction.html) or this page (http://www.ctio.noao.edu/~atokovin/tutorial/intro.html). [/slight derail]

[Brian the Snail]

Originally posted by bewareofdogmas
Easy proof that we went to the moon. this (http://www1.nasa.gov/vision/space/features/21jul_llr.html)

No, the shadow points in a different direction to the others.

Plus there's no stars!

sorry

[pgwenthold]

Originally posted by Brian the Snail
No, the shadow points in a different direction to the others.

Plus there's no stars!

sorry

Actually, I should point out that this fact does not derail hoaxers to any extent. They just claim that it was brought up by an unmanned mission. In fact, there is a Russian mirror on the moon, and they never sent people there.