
Why is the moon’s gravity so uneven? - rpledge
http://web.mit.edu/newsoffice/2013/an-answer-to-why-lunar-gravity-is-so-uneven-0530.html
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IvyMike
A mission to investigate the Tycho Gravitational Anomaly 1 would make a good
start for a book or movie. ;)

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ams6110
_gravitational fields resemble a bull’s-eye pattern: a center of strong, or
positive, gravity surrounded by alternating rings of negative and positive
gravity._

Negative gravity? Do tell....

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HCIdivision17
To be fair, the quoted sentence itself sets up the jargon used: "... a center
of strong, or positive, gravity ..." Think of it as setting up an analogy's
namespace.

Or just refer to it as gauge gravity, which is what they've described (where
zero is a reference to a nonzero absolute value).

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mike-cardwell
On a related topic. I read this the other day:

<http://www.bbc.co.uk/news/science-environment-22736709>

"The space rock, which is called 1998 QE2, is so large that it is orbited by
its own moon."

The rock is 1.7 miles across, and is orbited by another 600 metre rock. I find
it bizarre that a rock only 1.7 miles wide has enough gravity to be able to
have its own "moon".

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bennyg
It's all about mass. A neutron star's mass ratio would be like jamming 50
million elephants into something the size of a thimble. The article you
mentioned says almost 15% of large asteroids are a binary system. That's
pretty awesome to me. I had no idea about that, but I guess it's fairly
obvious.

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gus_massa
The relation between volume and mass for most solid in asteroids are:

* mostly water or ice: 1 g/cm^3

* mostly rock: 5 g/cm^3

* mostly iron or metals 8 g/cm^3

So density range of the usual solid material of asteroids is only ~10. This
asteroid has a very low gravity. So maybe jumping from it can put you in
orbit.

The easy way to understand what is imagining that you can turn off the gravity
force between the asteroid and its moon. Now you have two independent
asteroids that are orbiting around the sun. Both have almost the same position
and velocity, so both have a very similar orbit. They will travel together for
a long time, but the differences in the orbit will began to accumulate and
they will start to slowly come apart.

Now turn on the gravity force between them. It's a small correction to the
independent orbits but they are not separating very fast so a small force can
keep them together. If you stand up in the bigger asteroid then the bigger
asteroid will look like a moon.

(A better method to calculate the apparent orbit of the moon is to use a
reference system that is fixed to the center of the asteroid. It's
accelerating, so most of the gravity force of the sun cancels, but there are
some tidal and centrifugal effects.)

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leeoniya
i was about to say, wouldn't an uneven mass distribution large enough to throw
off probe trajectories also cause the moon to rotate off-axis until it
readjusts its shape? then i remembered that the moon doesn't rotate, maybe
precisely because of this!

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DanielRibeiro
It seems that the moon does rotate:
<http://curious.astro.cornell.edu/question.php?number=142>

At least to an observer sitting in a fixed point in outer space.

 _Edit:_ better explanation:
<http://www.badastronomy.com/bad/misc/moon_spin.html>

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leeoniya
okay, that's being a bit nitpicky, i meant with respect to Earth. but maybe
you're that guy who goes into detail about galactic rotation when someone asks
you to stand still for a photo :)

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deletes
Moon rotating has nothing to do with from where the observer is observing it.
An object is rotating if it produces centrifugal force.

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gizmo686
Circular motion can be thought of as analogous in many ways to linear motion.
Consider that you are on the surface of Earth, which is rotating. Therefore
you are also rotating, however relative to Earth, it makes sense to say that
you are not rotating. Similarly with the moon. If it were rotating relative to
Earth, then we would expect to be able to see what is on its current dark
side, as it would be the lit side after half a rotation.

Also, a rotating object does not need to produce or receive any force. The
conservation of angular momentum says that unless a force acts on a system, it
will continue to rotate at whatever angular velocity it is currently rotating
at. This is because the system itself is not accelerating. If you look at a
particlar subsection of the system (say at an end), then that subsection is
accelerating due to a centrifugal force, but that force is contained within
the system.

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ars
This isn't really accurate. Circular motion is absolute, but linear motion is
relative.

Even on the surface of the earth you can tell you are rotating, even inside a
completely closed room. For example with a
<http://en.wikipedia.org/wiki/Foucault_pendulum>

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aashaykumar92
Maybe an obvious answer, but why doesn't the Earth exhibit this? Or does it?

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panic
It does! See, for example, <http://en.wikipedia.org/wiki/Chicxulub_crater> \--
the asteroid that killed the dinosaurs left a very noticeable gravity anomaly,
which helped us identify the site as an impact crater.

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PySlice
Yeah, it's so annoying. Everytime I go to the moon I remember how I hate that
uneven gravity!

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dwc
Gawd, I _just_ dusted and look at it!

