To see the answer, go find a spinning computer chair. To make the principle as visible as possible here, try to find two equally heavy objects and hold one in each hand. Hold them far out to your sides, then start spinning, holding the objects there. Get a good spin going, get a feel for the speed of it, then slowly, still spinning, bring your arms in to your chest. The closer those weights get to your body, the faster you will spin.
One great thing about astronomy is the variety of sights it can provide. Do you want to see a star nested inside another star? You can see that. How about thousands of galaxies all colliding in an event so monstrously large and violent that the human brain can’t possibly comprehend it? You can find that, too. But try looking within any one solar system for instance: our own and you’re bound to find an orderly system: eight little soldiers all marching in lock-step, all circling the Earth and their own internal axes in the same uniform direction. But if the solar system was formed by the mostly random accumulation of gasses and astronomical debris, how could we come to such an ordered state of affairs here in our time?
This is a principle called the moment of inertia, and it has two major implications.
One: the distribution of mass in a system can exert some control over the spinning of that system.
Two: the spinning of a system can exert some control over the distribution of mass inside it. This means that as, say, a cloud of gas and space dust destined to become our solar system contracts slowly due to gravity, the angular speed of that spinning system will increase. Put even more simply: as the solar system was first coming together, its parts all started rotating faster and faster.
The actual direction of rotation was, in the beginning, utterly random. The material came in from all sides, from all directions, but eventually as these collisions started to add up to a cloud with enough gravity to hold itself together, the spinning chair principle came into effect. Inside the roiling, disorganized mass that would become our home in the galaxy, there was one direction of rotation with very slightly more mass behind it than the others and as the cloud pulled its arms toward its chest, that direction became more and more dominant. Particle speed lost to internal collisions between particles moving in different directions was offset by the increase in speed due to the overall contraction of the system.
By the time real bodies like stars and planets started forming, the physics were unstoppable. New matter entered the system at far too slow a pace to change the overall direction of travel; not only does every planet go around the sun in the same direction, but upwards of 99% of asteroids and other small features do too. The remaining less-than-1% can be surmised to be very new additions to our neighborhood.
The big, big exception to this is Venus, which orbits the Sun normally, but spins backwards relative to its own axis. Possible explanations range from a huge collision to a solar gravitational effect that slowed, stopped, and eventually reversed its rotation. Probably the most widely supported theory says that Venus actually is spinning the right way, and in fact simply flipped its poles and thus seems to be rotating the wrong way! We likely won’t know for sure until we can see through its atmosphere much more completely than we can today. Neptune also rotates on a tilted axis, which is also thought to be the result of a major ancient collision.
These principles of rotation are also why our solar system moves along a mostly flat plane; as one direction of spin came to dominate in the early solar system, the spinning mass naturally flattened out along that plane. Note that this plane of rotation in our solar system does not match up with the overall plane of galactic rotation; the direction of rotation within individual star systems is largely unaffected by the galaxy, but the systems themselves do all orbit the galactic core in one direction. It’s the same principle at work, but with whole systems this time.
The video below shows how an ordered system can arise from a disordered one, and it doesn’t even need to invoke the moment of inertia to do so.
One great thing about astronomy is the variety of sights it can provide. Do you want to see a star nested inside another star? You can see that. How about thousands of galaxies all colliding in an event so monstrously large and violent that the human brain can’t possibly comprehend it? You can find that, too. But try looking within any one solar system for instance: our own and you’re bound to find an orderly system: eight little soldiers all marching in lock-step, all circling the Earth and their own internal axes in the same uniform direction. But if the solar system was formed by the mostly random accumulation of gasses and astronomical debris, how could we come to such an ordered state of affairs here in our time?
This is a principle called the moment of inertia, and it has two major implications.
One: the distribution of mass in a system can exert some control over the spinning of that system.
Two: the spinning of a system can exert some control over the distribution of mass inside it. This means that as, say, a cloud of gas and space dust destined to become our solar system contracts slowly due to gravity, the angular speed of that spinning system will increase. Put even more simply: as the solar system was first coming together, its parts all started rotating faster and faster.
The actual direction of rotation was, in the beginning, utterly random. The material came in from all sides, from all directions, but eventually as these collisions started to add up to a cloud with enough gravity to hold itself together, the spinning chair principle came into effect. Inside the roiling, disorganized mass that would become our home in the galaxy, there was one direction of rotation with very slightly more mass behind it than the others and as the cloud pulled its arms toward its chest, that direction became more and more dominant. Particle speed lost to internal collisions between particles moving in different directions was offset by the increase in speed due to the overall contraction of the system.
By the time real bodies like stars and planets started forming, the physics were unstoppable. New matter entered the system at far too slow a pace to change the overall direction of travel; not only does every planet go around the sun in the same direction, but upwards of 99% of asteroids and other small features do too. The remaining less-than-1% can be surmised to be very new additions to our neighborhood.
The big, big exception to this is Venus, which orbits the Sun normally, but spins backwards relative to its own axis. Possible explanations range from a huge collision to a solar gravitational effect that slowed, stopped, and eventually reversed its rotation. Probably the most widely supported theory says that Venus actually is spinning the right way, and in fact simply flipped its poles and thus seems to be rotating the wrong way! We likely won’t know for sure until we can see through its atmosphere much more completely than we can today. Neptune also rotates on a tilted axis, which is also thought to be the result of a major ancient collision.
These principles of rotation are also why our solar system moves along a mostly flat plane; as one direction of spin came to dominate in the early solar system, the spinning mass naturally flattened out along that plane. Note that this plane of rotation in our solar system does not match up with the overall plane of galactic rotation; the direction of rotation within individual star systems is largely unaffected by the galaxy, but the systems themselves do all orbit the galactic core in one direction. It’s the same principle at work, but with whole systems this time.
The video below shows how an ordered system can arise from a disordered one, and it doesn’t even need to invoke the moment of inertia to do so.
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