Is the moon closer or further away to us than predicted by Newton

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Discussion Overview

The discussion centers around the accuracy of the moon's distance from Earth as measured by laser reflections from mirrors left on the lunar surface, particularly in relation to predictions made by Newton's gravitational formula. The scope includes theoretical considerations of gravitational models and the implications of experimental measurements.

Discussion Character

  • Exploratory
  • Technical explanation
  • Debate/contested

Main Points Raised

  • One participant mentions that laser measurements of the moon's distance may differ from predictions made by Newton's gravitational formula, though they are unsure if the moon is closer or further away.
  • Another participant argues that the question lacks clarity, stating that orbits can exist at various radii and that there is no specific radius predicted by Newton.
  • A participant points out that the average distance between the Earth and the Moon is increasing due to tidal gravitational interactions, which they claim is consistent with Newtonian predictions.
  • There is a humorous exchange questioning whether Apollo 13 left mirrors on the moon, with one participant confirming that it was actually other Apollo missions that left reflectors.
  • A later reply elaborates that while Apollo 13 did not leave reflectors, other missions did, and discusses the use of laser ranging to verify gravitational relations and search for general relativity effects.

Areas of Agreement / Disagreement

Participants express differing views on the clarity of the original question and the specifics of the gravitational predictions. There is no consensus on whether Newton's formula accurately predicts the moon's distance based on current measurements.

Contextual Notes

Participants note the potential confusion regarding which Apollo missions left reflectors on the moon and the implications of gravitational models, but do not resolve these points definitively.

seasnake
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I saw on t.v. that the moon's distance to Earth as measured by lasers reflecting off of the mirrors left on the moon by Apollo 13 was a bit off from Newton's Fg = Gm1m2 / r^2 formula. I don't recall if the moon was closer or further away from Earth than what was predicted by Newton's formula.
 
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I don't think this question makes sense. One can have an orbit at any radius, so there is no radius "predicted by Newton". There are differences between GR and Newton, but radius is not one of them.
 
seasnake said:
mirrors left on the moon by Apollo 13

Are you sure that Apollo 13 left mirrors on the Moon? :biggrin:

The average distance between the Earth and the Moon is increasing at the rate of almost two inches per year due to the tidal gravitational interaction of the Earth and the Moon, but, as Vanadium 50 posted, this is predicted by Newtonian gravity.
 
George Jones said:
Are you sure that Apollo 13 left mirrors on the Moon? :biggrin:

Tossed 'em out the window perhaps?
 
Stratosphere said:
Although Apollo 13 never left reflectors behind 3 other Apollo missions did.

Yeah, we know. Appolo 13 didn't leave anything on the Moon.
 
seasnake said:
I saw on t.v. that the moon's distance to Earth as measured by lasers reflecting off of the mirrors left on the moon by Apollo 13 was a bit off from Newton's Fg = Gm1m2 / r^2 formula. I don't recall if the moon was closer or further away from Earth than what was predicted by Newton's formula.
This is a very good question. Newton could have used his knowledge of the Earth's gravitational force in combination with the moon's sidereal period (Kepler's third law) to calculate the distance to the moon. Laser ranging could verify these relations. These laser measurements are now being used to look for GR effects. See
http://www.physics.ucsd.edu/~tmurphy/apollo/basics.html
In addition, click on "track our progress" at the bottom of this URL to see details of the laser ranging measurements.
 

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