The general theory of relativity considers not only frames of reference that are moving relative to each other at a constant velocity but also:
(a) those that have a relative acceleration and
(b) the effect of gravitational fields.
General relativity theory predicts the following:
(a) that light bends in a gravitational field - light just grazing the surface of the Sun has been observed to be deviated by some 1.75 seconds of arc
(b) that the perihelion of Mercury, (its nearest point to the Sun), shows precession
(c) that physical processes such as the vibrations within an atom are slowed down in a high gravitational field, and therefore the light coming from the stars is reddened slightly.
(d) that the effects of acceleration and gravitational field are equivalent