Why Does the Earth Orbit?

I know that the earth orbits the sun because the sun is pulling on it as the earth tries to move in a direction perpendicular to the pull. My question is: Why is earth so hellbent on moving in that perpendicular direction? Where did it get that initial push? From the initial movement of the gas and dust from which it was formed? If so, why was THAT stuff moving? Obviously my question applies to everything in orbit. There must be some common answer that I just don't know. Thank you.
- Paul Wagman (age 66)
Clayton

Hey Paul- 

It's really just sort of accidental. Things in the early universe were lumpy, just by accident. (A quantum mechanical accident, it seems.) You can still see that early lumpiness in the slight unevenness of the cosmic microwave background radiation, coming in from when the universe was only about 300,000 years old.

Gradually, gravity made the lumps get denser, since the densest spots pulled more things in. Like economics. Now look at one clump of stuff on its way to condensing into a star. There are neighboring clumps, pulling on it via gravity and maybe coming crashing through sometimes. Unless there's some special coincidence, those effects will be uneven on different sides, just by the accidents of where things are, leaving the ingredients of the star spinning.

Now as that spinning stuff clumps in further, it has to start spinning faster, just like an ice-skater pulling her arms in. That's conservation of angular momentum. But things that are spinning too fast cant collapse any further. So most stars either end up with some planets around them, carrying that accidental angular momentum, or split into two as a spinning binary star, carrying even more angular momentum. That's why all the Sun's planets orbit in about the same direction- it's all from that initial accidental angular momentum of the collapsing cluster of stuff.

We touched on this question briefly before: 

Of course, as all of us St. Louisans know, there's also the teleological explanation. Without a yearly orbit there would be no seasons. And without seasons, there would be no baseball seasons. What sort of universe would that be? (Sorry everybody else, inside joke.)

Mike W.

(published on 02/04/2015)