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Nevertheless, it has been verified with ever-increasing precision in subsequent eclipses, and in the observations of quasar 3C273.

Nevertheless, it has been verified with ever-increasing precision in subsequent eclipses, and in the observations of quasar 3C273.

The second "classical" test of general relativity was the advance of the perihelion of the orbit of Mercury.  There are many complex effects contributing to this, including gravitational perturbations from other planets and effect of the oblateness of the Sun.  These are hard to calculate accurately, but, by 1900 it was known reasonably accurately that there was an "anomalous" precession, that is, a precession beyond all other known effects, of about 43 arc seconds per century.  This is a very tiny effect, but astronomical measurements were sufficiently accurate by that time to show it clearly.  At first relativists claimed that the precession is explained by general relativity, but more accurate measurements revealed that general relativity is disproved by the observations of the precession.  Newtonian mechanics can explain the precession if the exponent of the distance in the force equation is not precisely 2, or if the tug on Mercury from other planets in the solar system is taken into account.

The second "classical" test of general relativity was the advance of the perihelion of the orbit of Mercury.  There are many complex effects contributing to this, including gravitational perturbations from other planets and effect of the oblateness of the Sun.  These are hard to calculate accurately, but, by 1900 it was known reasonably accurately that there was an "anomalous" precession, that is, a precession beyond all other known effects, of about 43 arc seconds per century.  This is a very tiny effect, but astronomical measurements were sufficiently accurate by that time to show it clearly.

 

This created quite a problem—physicists by then were accustomed to having their theories check out very accurately.  One proposal that was floated, by Simon Newcomb, was that the exponent of the radius in the gravitational formula wasn't exactly 2.  By choosing a value of 2.000000157, Newcomb was able to fit the observed behavior of Mercury.  Unfortunately, any such fudging of the exponent gives the same precession, per planetary year, for ''any'' orbiting object.  This would have given the Moon a very noticeable precession, which was not observed.

 

General relativity predicts this extra precession, in the amount of <math>3 v^2/c^2</math> revolutions per planet's "year", where <math>v</math> is the planet's average orbital speed.  There is some controversy over whether general relativity's results are really accurate in terms of modern observations.

[[Image:Cassini-science-289.jpg|right|thumb|signal dipping into a gravity well around the [[sun]]]]

[[Image:Cassini-science-289.jpg|right|thumb|signal dipping into a gravity well around the [[sun]]]]