Originally Posted by

**Ken G**
That's true, but the point is, there are (at least) three very different ways to explain an orbit of a point-particle (our approximation to the astronaut), and the typical physicist learns all three:

1) Newton: There is exactly one real force on the orbiting astronaut, and it is gravity, and it causes the astronaut to accelerate, which he/she needs to do in order to go in a circle.

2) Einstein: There is exactly zero force on the orbiting astronaut, so he/she does not accelerate at all, but rather follows the inertial path determined by gravity and their orbital speed, and that path is a circle.

3) d'Alembert: There are exactly two forces on the astronaut, one gravity and one the inertial force (a specific case of which we call the centrifugal force), and those forces balance in the frame of the astronaut. Everything around them will appear to accelerate in a circle around them, but the astronaut cannot perceive acceleration of their own frame, it is their own frame.

Each of these pictures has its strengths and weaknesses. Only #2 is accurate for strong gravity, but all three work well for weak gravity. Above all, though, we must not imagine that any of those three languages is "correct", for each are known to fail in some context.