Answer to Question #37438 in Mechanics | Relativity for Renaud Roy

Question

In chapter 4 of Einstein's "Relativity", entitled "The principle of relativity (in the restricted sense)", it says that the principle of relativity must apply with great accuracy in the domain of mechanics, because it supplies us with the motion of the heavenly bodies with a delicacy of detail little short of wonderful. How is the principle of relativity related to the motion of the planets if they are not in inertial reference frames (e.i. they are not travelling in a constant velocity and in a straight line)? What am I missing? Thank you!