Re: In the Shadow of the Moon

In sci.space.history message <JJpsCI.Kno@xxxxxxxxxxxxx>, Sat, 16 Jun
2007 05:53:06, Henry Spencer <henry@xxxxxxxxxxxxx> posted:

The L4 and L5 points, aka "Trojan" points -- 60 degrees ahead of and
behind (e.g.) Earth in its orbit around the Sun -- *are* stable, if
perturbations from other bodies are negligible (not always a safe
assumption). But L1, L2, and L3, the "in-line" points, are not.

Strictly, L4 and L5 are not in Earth's orbit around the Sun, taking that
as defined by the centre of the Earth.

L4 and L5 each form an equilateral triangle with the centres of Earth
and Sun; however, the Earth does not orbit the centre of the Sun but the
Earth-Sun barycentre.

Diagrams at <URL:http://www.merlyn.demon.co.uk/gravity4.htm> for
circular orbits; I believe the elliptical case is much the same.

Moreover, contrary to what NASA has said, L3 is not further from the
primary than the secondary is - though it is further from the
barycentre than the secondary is.

--
(c) John Stockton, Surrey, UK. ?@merlyn.demon.co.uk Turnpike v6.05 MIME.
Web <URL:http://www.merlyn.demon.co.uk/> - FAQqish topics, acronyms & links;
Astro stuff via astron-1.htm, gravity0.htm ; quotings.htm, pascal.htm, etc.
No Encoding. Quotes before replies. Snip well. Write clearly. Don't Mail News.
.

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