Rotation of Galaxies, stars, planets etc.

feckthepeas

Hi Guys,

I'm sure this is a probably the question of a simpleton but I don't quite understand why galaxies, planets and stars rotate. I read that rotation is caused by angular momentum, which I must admit to finding a bit hard to understand...

"Angular momentum is given by L=m*w*r2 where m is the mass, w is the angular velocity in radians per second, and r is the radius of the circular motion. Due to conservation of angular momentum, if the radius of the orbit decreases, then its angular velocity must increase (as the mass is constant)"

What I'm wondering is, do galaxies all spin in different directions in relation to each other? Why does the Earth spin anti-clockwise on its axis, while Venus spins clockwise. And if we also orbit the sun anti-clockwise, why does Venus do it too, if it spins on its axis in the opposite direction to us? Does the direction in which we spin on our axis have any bearing at all to the direction in which we orbit the sun?

FISMA

Physicists and Astronomers don't really do "why."

The question - "why" do objects rotate is for Philosophy/metaPhysics.

Objects do move along curvilinear paths in the Physical world. This is the interesting observation.

Next comes mathemaztion so that we may predict the outcome of the Physical system.

I would expect that the different rotations would be theorized to have occurred as a consequence of conserving angular momentum during collisions that formed and shaped the Universe.

feckthepeas

FISMA wrote: »

Physicists and Astronomers don't really do "why."

The question - "why" do objects rotate is for Philosophy/metaPhysics.

Objects do move along curvilinear paths in the Physical world. This is the interesting observation.

Next comes mathemaztion so that we may predict the outcome of the Physical system.

I would expect that the different rotations would be theorized to have occurred as a consequence of conserving angular momentum during collisions that formed and shaped the Universe.

Forgive my ignorance, but why would it be a question for philosophy? Perhaps I phrased myself badly, what I'm asking is how it happens. What sets these bodies in motion?

Can you explain 'angular momentum' in simple terms for me, this is the part that I need spelt out I suppose.

3DataModem

What set the bodies in motion?

The big bang, ultimately.... followed by many millinia bouncing off each other, and other bodies' gravitational forces, and... that's about it.

feckthepeas

But what about newly forming stars, they rotate too. Is it that the clouds of hydrogen from which they are formed are already rotating? Am I just being painfully stupid?

3DataModem

Nope not stupid.

All good questions.

And yes, you are right.

Universe has lots of energy in it (from big bang, God, whatever) and a lot of it is manifested in spinning things, explosioning things, and attracting stuff together.

feckthepeas

Thanks for that! So it's just a random occurence that Venus happens to be spinning on its axis in the opposite direction to the Earth, there's no more to it than that?

tricky D

Galaxies' spins are generally independent of each other. As to why some spin clockwise or anti-clockwise, they don't really as it depends on whether you're looking from above or below.

Why do galaxies and solar systems spin? Spinning increases stability. Ones that don't spin tend to break up or not really form due to lack of stability.

Why do the planets' orbits all rotate the same way? When the solar system is forming it's a disc of matter spinning in the same direction due to the stability requirement. The direction is maintained as the planets form due to accretion of matter.

Venus spinning in a different direction is possibly due to it being hit by something resulting in a reversal of direction. We were likely hit when the Moon was created but our rotation wasn't changed.

djhaxman

feckthepeas wrote: »

Thanks for that! So it's just a random occurence that Venus happens to be spinning on its axis in the opposite direction to the Earth, there's no more to it than that?

Here's another one for you. Venus spins so slowly on it's axis, that a day is longer than a year.

JuneBug29

FISMA wrote: »

Physicists and Astronomers don't really do "why."

The question - "why" do objects rotate is for Philosophy/metaPhysics.

Firstly, I think the OP's question is valid and I think it's wrong to assume they have no knowledge. You seem to have taken this stance.

Secondly, Isn't the "Why?" question not the basis of every branch of science?

slade_x

feckthepeas wrote: »

Hi Guys,

I'm sure this is a probably the question of a simpleton

It most certainly is not. related questions are, whats give matter its mass, what causes or what primordial force or occurance caused gravity. Gravity is the least understood force. Understanding it fully still alludes us, but discovering its mysteries will unlock the secrets of our cosmos

feckthepeas wrote: »

Why does the Earth spin anti-clockwise on its axis, while Venus spins clockwise. And if we also orbit the sun anti-clockwise, why does Venus do it too, if it spins on its axis in the opposite direction to us? Does the direction in which we spin on our axis have any bearing at all to the direction in which we orbit the sun?

Angular momentum describes the state of moment of a physical system. be it individual particles or a solar system. there is no force driving it. it exists as an observation of how a system behaves.

The Earth spins because of the angular momentum conserved in its formation from the original gas cloud (nebula) that our solar system formed from, which in turn came from our spinning galaxy, etc. The cloud which was many orders of magnitude larger than our formed solar system collapsed resulting in a huge change in the size of the system. you can see this in action with the example of the spinning ice skater, a tornado and even neutron stars.

As the collapsing cloud formed a protostellar disk the outer part of the disk had a different rate of rotation as its inner segments

Its easy to experiment with this without needing to create your own solar system. if you have a record player for example, placing something on the record closer to its center will result in that object completing more rotations a minute than an object on the very edge of the record. if the record was to shrink , well say the energy or momentum from the force driving the outer part furthest away from the center would be conserved as it shrunk, the system doesnt lose its energy, its angular momentum remains the same. of course if a system rotates too fast it could very well destroy itself but theres where gravity comes in. a rotating object rotating fast enough would rip it self apart if not for gravity.


The forces that forged our universe are responsible for the initial momentum that allows a solar system like ours to exist.

And the accepted theory for venus' retrograde rotation is that it was struck by a substantial asteroid in its very distant past which hit it with enough force and angle to cause it to rotate in the opposite direction, the same kind of event is theorised to explain uranus also. Angular momentum is what kept these planets orbiting the way they do as they are all part of the original system.

It would take a truly devastating impact for a body to be hit with enough kenetic energy at exactly the right angle to effectively reverse its orbit in such a system. that kenetic energy would have to be delivered by either a body with mass much much much greater than that of its own or with a mass the same or less accelerated to a truly incredible speed, a speed much much greater than any asteroid we have ever observed. It would have to also not originate from our solar system for the very reason of conservation

slade_x

FISMA wrote: »

Physicists and Astronomers don't really do "why."

The question - "why" do objects rotate is for Philosophy/metaPhysics.

.

The question why is for any and all to endeavour to answer.

Galileo pondered the "why". he was a physicist, an astronomer, a mathematician and a philosopher.

Modern astronomy is much more than the cataloging of celestial bodies and phenomena. it encompasses a much broader field of study today. And Astronomers almost certainly ponders the why. The awe of the hobby is all in the why and the how.

Thats too much of a generalisation. The why is the goal of a lot of fields of study. look at Cern for example

Kevster

feckthepeas wrote: »

What I'm wondering is, do galaxies all spin in different directions in relation to each other? Why does the Earth spin anti-clockwise on its axis, while Venus spins clockwise. And if we also orbit the sun anti-clockwise, why does Venus do it too, if it spins on its axis in the opposite direction to us? Does the direction in which we spin on our axis have any bearing at all to the direction in which we orbit the sun?

I'm no expert in this area dude. However, think of it like this: if you set a planet moving in one direction in space, it'll continue to move in that same direction indefinately (unless it collides with another or is affected by a strong-enough gravity field). The directions that all of the planets currently orbit the Sun in - and indeed the directions that each planet rotates in - are all due to the way the solar system was formed and any collisions that happened thereafter.

I'm not sure why Venus spins in the opposite direction, but it could be something as 'simple' as it having collided with a larger object in the past. It's broken remains then accumulated together again in such a way tha the planets rotation was in the opposite direction. One must look at Uranus too, which rotates on its side. This might have been due to a collision too.

Think of planets as marbles.

Kevin

feckthepeas

Thanks all, think ye've clarified that for me nicely!

Gurgle

Kevster wrote: »

I'm not sure why Venus spins in the opposite direction, but it could be something as 'simple' as it having collided with a larger object in the past.

I've heard / read 3 different answers to that question.
- A glancing collision
- 'Tidal' effect caused by the other planets in the solar system
- Weather, billions of years of high intensity storms