Changes

===Orbits===

===Orbits===

General relativity provides one explanation for the precession of Mercury's perihelion, which was moving at a different speed than that predicted by a simple application of Newton's law of universal gravitation.  (Previous scientists had attempted to explain it by the gravitational pull of a hypothetical planet inside Mercury's orbit, which they called [[Vulcan]].  This could also be explained by altering the precise inverse-square relation of Newtonian gravity to distance, but that was disfavored by mathematicians due to its inelegance in integrating.)

General relativity provides the best known explanation for the precession of Mercury's perihelion, which was discovered in the 1840s to be following an orbit that was slowly precessing (rotating), rather than being fixed in space as Newtonian gravitation requires.  Actually, the effects of other planets on the precession has been known for a long time, but, when those effects are accounted for, there was still an "anomalous precession" of 43 arc seconds (an extremely tiny amount) per century.  Previous scientists had attempted to explain it by the gravitational pull of a hypothetical planet inside Mercury's orbit, which they called [[Vulcan]]; no such planet was found.  It could also have been explained, by a hypothesis put forward by Simon Newcomb and Asaph Hall, by altering the precise inverse-square relation of Newtonian gravity.  That theory was discarded, in the very early 20th century, when it was found to give wrong values for the orbit of the Moon.

===Gravitational Lensing===

===Gravitational Lensing===