Posted by Bob on 10/31/24 11:28

On Wed, 5 Oct 2005 18:24:17 +0100, Paul Hyett
<pah@nojunkmailplease.co.uk> wrote:

>I know the Apollo astronauts left reflectors for laser distance
>measurement.

Yes, very large and very expensive corner reflectors.

>How they work out its moving away at 1.5" a year is a
>mystery though, since its distance from the Earth changes constantly
>because of its elliptical orbit

If you track this elliptical orbit precisely and it shifts 1.5" per
year, then you can say the Moon has moved 1.5" inches farther from
earth.

BTW, the actual orbit of the Moon is hardly elliptic. The Moon, Earth
and Sun constitute a 3-body system and that means the orbit is not an
ellipse. Furthermore the Moon is influenced by the other planets.
These perturbations result in a non-linear equation for the orbit
which can be measured with laser interferometry.

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