Why is the moon’s gravity so uneven? | Hacker News Reader

Why is the moon’s gravity so uneven?

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58 pointsrpledge26 comments

A mission to investigate the Tycho Gravitational Anomaly 1 would make a good start for a book or movie. ;)

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....

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).

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".

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.

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.)

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!

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

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 :)

Sorry, it was not meant to sound nitpicky. I was surprised that I actually had never stopped to think about Moon's rotation, and that maybe it did not spin to a stationary observer.

The more you know...

As long as you're onto spinning things, check out Mercury's rotation.

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

Or rather, the other way round...

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.

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

Objects have a tendancy to syncronize their rotations. Additionally, what I understand to be, the leading theory on how the moon was made is that it was blown off the earth, in which case conservation of angular momentum should make it start not rotating relative to earth.

"Should make it start not rotating" - what does that sentence even mean?

Sorry, that should read "not start rotating". Essential its Newton's first law, unless there is a force that would make the moon rotate relative to earth, the moon will not do so.

The moon does rotate, but the orbit is synchronised such that one rotation occurs per orbit, so from the Earth it appears not to rotate. However, if you photograph the moon over time you find you can see >50% of the surface area (about 60% I think) due to libration. https://en.wikipedia.org/wiki/Libration

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

It does, but a lot less: http://en.wikipedia.org/wiki/Geoid

The size difference would be the main reason. On earth, the dense material is much deeper, so you'd need a lot more force to bring the it to the surface. The increased size and soft earth probably absorb more of the shockwaves as well. Having no atmosphere means that even smaller impacts will reach the surface as well.

There are probably some impacts that have caused this on earth, but the difference in gravity is probably so small, that it doesn't effect orbits. (We could probably measure it though).

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.

Earth's radius is 3.7 times moon's; volume is 54 times. This difference may translate to the difference in phenomenon.

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

Gawd, I just dusted and look at it!

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