What determines "rotation"?

[Synthesiz]

Okay, now this may seem a little.. philosophical, but it's a question I have been asking myself for years, and haven't so far found an answer to.

 

Say we have an imaginary universe with only two reference points, one star and one planet in it. Furthermore, from the perspective of the planet, it appears as if the star is rotating. So far so good. But now imagine there's a human on the planet with a telescope, and he asks himself: "Am I going to fall into the star?".

 

Given the information we have here, I think it's impossible to answer. The planet might be orbiting around the star, or the star might be revolving around itself while the planet is stationary. In the first case, the planet will stay at the same distance. In the second case, the planet is going to fall into the star. In other words, if we just span a coordinate system between our two planets, that won't give us the full story. We also need to make sure this coordinate system has the "right rotation".

 

And now my big question: How do we "find" such a coordinate system with the "right rotation"? What do we use as reference point? The solar system? But what if the solar system rotates, too? Does it even matter whether it rotates or not? This all has me very confused.

 

Hoping to get some answers that an amateur like me can understand. ^^

[granpa]

rotation is not relative.

rotation is absolute

[swansont]

As granpa said, rotation is absolute — it's an acceleration. You can tell if something is accelerating. Newton's laws tell us that an object that is not accelerating will move in a straight line at a constant speed. If you see a change in direction or speed, you know there is an acceleration.

[Enthalpy]

Rotations are absolute. You can detect them through the Coriolis force.

You can distinguish them from a gravitation field if you compare the movement of free masses at different positions.

Put a mass nearer to the star than the planet is, it moves forward; put an other one farther, it moves backwards. This tells you the planet orbits the star.