 Indeed, the analogy between the two kinds of case helps to explain Newton’s changing the relativity principle from a law to a Corollary, for this coincides historically with his first use of Corollary VI. The two Corollaries identify two classes of frames of reference that may be treated as equivalent, because they involve, respectively, theoretically and practically indistinguishable states of motion. The frames corresponding to Corollary VI may be called “quasi-inertial,” because “approximately inertial” might be misleading: a closed orbit around the Sun—like that of Jupiter’s system—is not a good approximation to an inertial motion, and the system can hardly be considered isolated. But over sufficiently limited segments of its orbit, its motion is sufficiently close to being inertial. Moreover—and most important—the accelerative forces toward the sun are close enough to being equal and parallel that the forces acting within the system can be effectively isolated from the forces from without. Hence, while “quasi-inertial” is a useful term for the reference-frame corresponding to such a group of bodies, a useful description for the group itself is George Smith’s “quasi-insular system” (Smith 2019). A system of masses bound in orbit around a larger mass is by no means isolated, yet in the right conditions it may be treated as if it were. The modern term, “local inertial frame,” is not inappropriate (cf. Schutz 1990, p. 124). But it typically designates the local coordinate frame of a given inertial observer, rather than the sort of “whole space” that Newton had in mind, as encompassing a celestial system as large as that of Jupiter, or the solar system as a whole. Moreover, it is typically used in a context in which a global inertial frame, with respect to which any such Newtonian system has a definite acceleration, would not be assumed to exist. 