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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.'"
Same way we ever check, have another group do an indepenent measurment or make them show their data and analysis?
Force = Weight.
the formula for Force is: F=ma, replace a with the acceleration caused by gravity and you got weight.
Only dead fish swim with the stream...
This has already been done, years ago. I can't recall the exact reference, but it was one of the examples used in calc class. Interesting stuff, but how about some modern news?
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.
Patrick Doyle
I mod down every jackass who puts his moderation policy in his sig. Oh, wait a sec....
- 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
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 :)
0 0_reflectors.htm for some pics...
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/tps50
(1) Before we can even discuss basic science research, we need to agree that such fundamentals are important and thus deserve tax dollars. I find it irrelevant that the money comes from NASA for this discussion.
(2) As mentioned in the article, this experiment hopes to measure to unprecedented accuracy the rate of change of the distance between the Earth and the moon. Why is this useful? If it can be done accurately (the conditions of which I will discuss in a moment) it would allow a determination of the self-interaction of gravity, e.g. graviton-graviton interaction. This is fundamentally different than Newtonian gravity, and, as mentioned elsewhere, the simplest way to explain in our nascent theory of quantum gravity the Einstein field equations without solving the actual math. On a much larger scale, the determination of the Hubble constant and how it changes with time also measures this. Finding the argument that it is preferable to do such measurements in one's backyard when possible I leave as an exercise to the reader.
With respect to the potential accuracy (vs. precision) of such measurements I will note the following. First, current gravity meters based upon atomic fountains are accurate enough to find Cave complexes in Afghanistan and see people moving around in them. (c.f. Steve Chu's recent work at Stanford with atom interferometers); we have a very detailed picture of our local gravitational field available to us. Second, considerations such as chaos theory and effects of the other planets are relatively straightforward to deal with. Back at the beginning of the 20th century they had already done it for Mercury and still had a discrepancy, at 43 arcseconds per century in its orbit(c.f. this explanation). That's over 10 times smaller all the other planets' influence, and that was calculated before computers. It seems to me the greatest unknown is the tectonic structure of the moon and the associated vibrations in the mirror. I suppose that radar rangefinding, given the scale of these variations, would be sufficient for most purposes.
Finally, some of the past results of this experiment, from the Nasa site
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.
For a much more rational and thought out theory on the origin of "fossil" fuels and other hydrocarbons see Thomas Gold's website.
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.
bah
The moon isn't all that close. Even Hubble can only get a resolution on the order of 100-meters or so. Pretty impressive, but far too large to discern a lander.