of gravity is stronger the closer you are to the sun, so you'd think that if the earth

got bumped slightly towards the sun (like, if you jumped, or something), then the sun

would pull just a little bit stronger, tugging the earth yet closer still, at which point

the sun's pull would be stronger yet again, … and the earth would spiral in to its eventual

doom.

But you and I and swiss cheese are clearly evidence that the earth doesn't just spiral

in to its doom – so why not? Well, when the earth does get pushed a little bit towards

the sun, the sun's pull speeds it up a little bit, too - so even though the nearby sun's

pull is stronger, the earth will be going fast enough that it overshoots and ends up

farther away from the sun. Of course, the sun's pull slows it down again in the process…

at which point the earth turns back and starts the cycle all over again.

The amazing thing is that the two effects of speeding up plus stronger gravitational

attraction perfectly balance, so that the earth stays in its orbit the same way a marble

stays at the bottom of a bowl even if it gets knocked around a little bit.

This balance is very special indeed - it's highly dependent on the strength of the gravitational

force and the number of dimensions we live in, and in fact, these stable orbits only

exist in a three dimensional universe.

The details are a bit subtle, but if gravity were just slightly steeper, as it would be

if we lived in four spatial dimensions, you could never orbit the sun because you'd get

pulled in without picking up enough speed to escape and you WOULD spiral in to your

doom. And if we lived in fewer dimensions so that gravity were a bit tamer, then you

could ALSO never orbit the sun, because as you approached it, you wouldn't be pulled

enough and would merely pass by with your path slightly bent.

So we're incredibly lucky to live in a three dimensional universe where there ARE stable

orbits that allow planets, solar systems, and galaxies to exist at all! Thank you, stable

orbits, for without you, life as we know it, and especially cheese, would be impossible.

ps An awesome down-to-earth example of stable orbits are the hyperbolic funnels that you

often see at museums or in shopping malls: the physics is almost identical to the stable

orbits of planets, satellites, and moons, and the only reason the coins in the funnels

(or satellites around the earth) eventually DO spiral in to their doom is because they

lose energy to friction and thus shift down from one stable orbit to another, until eventually,

their orbit coincides with the ground. Which is called crashing.