Caption: The Alien in a free-falling elevator and a free-falling spacecraft in orbit. Both are free-falling in the sufficiently uniform external gravitational field of the Earth.

The Alien is weightless in the elevator and spacecraft because he is free-falling with those containers and there is negligible internal gravitational field.

If you attach reference frames to the two systems unrotating with respect to the observable universe: i.e., to the bulk mass-energy of observable universe) (see Wikipedia: Inertial frame of reference: General relativity), those reference frames are inertial frames.

Now the center of mass of the Earth is also in free-fall, and so an unrotating reference frame attached to it, is also an inertial frame.

The observable universe contains a whole hierarchy of inertial frames attached to the centers of mass of astronomical objects.

Yours truly calls them celestial frames.

Credit/Permission: © David Jeffery, 2017 / Own work.
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