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

  • Level: High School 
  • Thread starter Thread starter seasnake
  • Start date Start date
  • Tags Tags
    Moon Newton
Join the discussion
Registration is free. Start your own thread to ask a follow-up.
Sign up Log in
6 replies · 3K views
seasnake
Messages
41
Reaction score
0
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.
 
Astronomy news on Phys.org

Discussion

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.