13.2.1—That, for a point outside a uniform sphere, the mass of the sphere may be

For points outside a spherically symmetric mass distribution, gravity acts as if the total mass were concentrated at the sphere’s centre.

Use centre-to-point distance r, not distance from the surface, when applying point-mass equations.

The external gravitational field of a spherical planet can be calculated with its total mass and the distance from its centre.

The point-mass equivalence does not apply inside a nonuniform shell or to an arbitrary irregular body.