Moon questions

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SF

Main Question or Discussion Point

Found this in another forum:

1) Is the moon really orbiting the earth?

The sun already has greater gravitational effect (stronger force) on the moon than the Earth does. (If you do not believe, calculated the relative Mass/separations^2 ratios for both sun and Earth (except for factor "G", the gravitational force at the moon).

In fact, the moon is now going arround the sun in essentially the same elliptic orbit as the Earth is. I.e. The moon only appears to be going around the Earth when viewed from the Earth. In fact, it is always with a trajectory that is curving towards the sun. Never is the moon on a trajectory that is convex towards the sun (as you would expect it to be if it were going around the Earth like most man made satellites are).

If the Earth were magically removed there would be very little difference in the moon's orbit about the sun. That orbit now has slight
"wobbles" about the true ellipse, which are caused by the relatively weak gravity from Earth and of course if Earth disappeared or had no gravity, they would cease to exist, but viewed from Mars, it would not be a noticable change in the moons orbit if the Earth had zero gravity. Earth does NOT control the moon. Really Earth does not have a natural moon, but is the larger of two interacting masses that co-orbit the sun.

Thread is badly named, reflecting this common erroneous POV. Should be called "Sun's planatoid near Earth" or something like that.
2) What is the effect of the Universe's expansion on the Moon/Earth or Sun/Earth distance?
http://www.news.cornell.edu/stories/March06/CMB_Timeline300.jpg [Broken]
From that image, there's been about 10-20% universe expansion during the last 4 billion years.
Does that translate into any effect on the "small" distances in our Solar System?

Thanks.
 
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1) Yes the moon is orbiting the Earth, but very slowly (it only goes around the Earth about 12 times a year), so it's true to say that the motion about the sun is the dominant factor in the Moon's motion in the solar system. In a similar way, the Sun's motion around the Galactic Centre is greater than the Earth's motion about the Sun, but it's still true to say the Earth orbits the Sun.

2) The expansion of the universe is negligibly small on the scale of the solar system - the Earth's orbit about the sun should grow something like a few nanometres per hour. The force of gravity is more than strong enough to overwhelm such a minuscule perturbation. Only in the vast open spaces of the universe where gravity is negligibly weak does the expansion manifest.
 

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