

The Orbit of the Moon around the Sun is Convex - ColinWright
http://www.math.nus.edu.sg/aslaksen/teaching/convex.html?HN_2

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cmurphycode
Wow.

I understood the argument that the speed of the Earth's orbit dominated the
Moon's, but I was having a really tough time visualizing it. The wiki article
([http://en.wikipedia.org/wiki/Orbit_of_the_Moon#Path_of_Earth...](http://en.wikipedia.org/wiki/Orbit_of_the_Moon#Path_of_Earth_and_Moon_around_Sun))
has a diagram that explains it
(<http://en.wikipedia.org/wiki/File:Moon_trajectory1.svg>)

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gort
Another argument is this:

The (wrong) graph has been drawn with 12 cycles, based on approximately 12
lunar months per year. But the result is obviously not to scale. If you try to
draw it to scale, the truth jumps out at you; it can't have this form.

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praptak
Yeah, this fact makes it awfully hard to draw realistically. Just try to draw
a circle (Earth's orbit around the sun) and then a convex curve that crosses
it from time to time. Pretty hard not to make it concave.

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Sharlin
Given the Earth-Moon distance of about 380000 km and Earth-Sun distance of
about 150000000 km, the lunar orbit deviates from Earth's orbit about 0.25% at
most; if the Earth-Moon distance [1] were one meter, the maximum deviation
would be ~2.5 millimeters. So, yeah, "awfully hard" just about describes the
situation.

[1] The average distance - Earth's orbit is not a circle, after all.

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wahnfrieden
Does the fact that the earths orbit is not a circle lead to an instability? So
the deviation will change over time - whether by converging on a circle or
spinning away. [totally naive question]

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iwwr
The Moon is slowly moving away due to tidal interaction. If the Earth and Moon
were tidally locked (both presenting the same face to each other at all times)
then the orbit would remain stable. If the Earth would be spinning slower than
the Moon, then the Moon would be slowly pushed inward.

An interesting consequence is that in a billion year's time, after Earth's
oceans would have evaporated, this tidal interaction would greatly reduce,
"fixing" the Earth-Moon system in place.

More here: <http://www.talkorigins.org/faqs/moonrec.html>

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wahnfrieden
Thanks, very interesting.

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euroclydon
The spirals don't seem intuitive to me. Do most people really think the moon
has a negative velocity during parts of its orbit about the sun?

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jdpage
Why wouldn't it? When it's going around the earth, it's going to be going in
the opposite direction from the earth's orbit part of the time. The question
is which motion is bigger.

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josyula
I thought it would look like a sine wave on a circle...Well but that totally
unexpected. Do check the guys other writings they are really __very
interesting __.

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sambeau
Spirograph is a lie!

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sixwing
this isn't a shaggy dog joke about the moon having an orbit closely conforming
to a polygon with 13.37 (1337) sides?

