Why CCW?

[specialopsdave]

Does anyone know why celestial bodies tend to orbit it's parent in a counter-clockwise fashion relative to the parent's north pole?

 

[peadar1987]

Conservation of angular momentum of the original cloud of matter that coalesced to form the solar system

[rudi1291]

A few weeks ago i asked myself the same and i found this paper: http://mnras.oxfordjournals.org/content/388/4/1686

It looks like our galaxy just happens to spin counter-clockwise, and that defines the direction of spinning for all celestial bodies in our galaxy

[specialopsdave]

But... Why was IT moving Counter-Clockwise?

[K^2]

7 minutes ago, specialopsdave said:

But... Why was IT moving Counter-Clockwise?

Erm... What do you mean? If you look at it from the other side, it's moving clockwise. And it has to spin one way or another, or it will collapse.

[Scotius]

16 minutes ago, specialopsdave said:

But... Why was IT moving Counter-Clockwise?

Because of our anthropocentric (and only one we have) frame of reference.

[specialopsdave]

12 minutes ago, Scotius said:

Because of our anthropocentric (and only one we have) frame of reference.

I thought we used Magnetic North as a point of reference.

[specialopsdave]

Another question: How did the Moon (the real one) get into an elliptical orbit of Earth? If the impact slowed it from escape velocity, It'd be on a suborbital trajectory. What, did it do, like, get a gravity assist from a passing asteroid, or what?

[peadar1987]

18 minutes ago, specialopsdave said:

Another question: How did the Moon (the real one) get into an elliptical orbit of Earth? If the impact slowed it from escape velocity, It'd be on a suborbital trajectory. What, did it do, like, get a gravity assist from a passing asteroid, or what?

It wasn't ejected as one big lump, it would have been a huge amount of debris. A lot of that would have fallen back to earth, the rest, through collisions and gravitational interactions, coalesced to form an orbiting moon.

[specialopsdave]

Ah, Okay! (You can't blame me for not knowing that: I'm a chemist, not a physicist.) Thanks!

 

[pincushionman]

On July 17, 2016 at 8:03 AM, specialopsdave said:

I thought we used Magnetic North as a point of reference.

North and South have been known since before magnets were widely used as compasses, at least as a concept. And which one is "up" or "down" us arbitrary; in many medeaval European maps "up" was East (kinda), toward Jerusalem.

So, at some point convention decided that "up" was such that the Earth rotated counterclockwise (note that the way clocks rotate…is ALSO arbitrary!). Earth inherited both its rotation and revolution (orbit) from the Solar system's progenitor gas/dust cloud. Since the rest of the solar system bodies also came from this cloud, they largely have the same angular monentum direction, too.

…I was about to extend that example to the Solar System inheriting the Milky Way's angular momentum, but then I realized that inter-star interactions could potentially jack up that trend when it cones to galactic orbits.

The weirdness comes about with exceptions to the trend. Moons with odd angular momenta can be explained well enough with ejection/capture mechanics, but Uranus…just…Uranus. Something really exceptional happened there. And this where the arbitrary distinction comes in - which way us North there? If you define North as "most consistent with the Solar System's/its own orbit's andular momentum", then its axial tilt is almost 90° AND it rotates clockwise (backward). BUT if you instead define North as "such that a body rotates counterclockwise when viewed from that direction," then its axial tilt is MORE than 90°, but it otherwise rotates the same as everyone else.

Edited July 18, 2016 by pincushionman

[Snark]

"North" and "South"-- like "clockwise" and "counterclockwise", or "left" and "right"-- are arbitrary human concepts, just labels.  They're defined in terms of one another.

You could simply say that the "north" pole of a body is the one around which the body rotates counterclockwise, by definition.  So in such a case, asking "why is it always counterclockwise around the north pole?" would be essentially meaningless-- it would be like asking "why does the sun always rise in the morning?"  (Note my use of conditional language, here.  AFAIK there's no universally accepted convention of what "north" is.  I happen to like that definition myself, but not everybody uses it for everything.  For example, consider the terminology of "north" and "south" for the planet Venus, which rotates in the opposite direction to most other planets-- it gets messy.)

Now, what's not a meaningless question is, "why is it that bodies almost always seem to orbit their primary in the same direction that the primary is rotating?"  Also, "why is it that bodies almost always seem to rotate on their axis in the same direction that they revolve around their primary?"

The answer to that one is, as @peadar1987 points out, conservation of angular momentum.  The original cloud of gas and dust that formed our solar system happened to have some slight rotation to it.  As it collapsed under its own gravity and coalesced into the sun and planets, they kept that same rotation.  Since they all formed from the same cloud, they ended up rotating in more or less the same direction.

There are some outliers-- Uranus is tipped on its side, for example, and Venus is hardly rotating at all (and what little rotation it does have, is retrograde).  But that gives the general gist.

[PB666]

On 7/17/2016 at 7:32 AM, rudi1291 said:

A few weeks ago i asked myself the same and i found this paper: http://mnras.oxfordjournals.org/content/388/4/1686

It looks like our galaxy just happens to spin counter-clockwise, and that defines the direction of spinning for all celestial bodies in our galaxy

If you flip the galaxy over and view from the other side which direction does it spin?