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According to Newtonian mechanics and Newton's law of gravitation,

a planet which is revolving round the sun would describe an

ellipse round the latter, or, more correctly, round the common

centre of gravity of the sun and the planet. In such a system,

the sun, or the common centre of gravity, lies in one of the foci

of the orbital ellipse in such a manner that, in the course of a

planet-year, the distance sun-planet grows from a minimum to a

maximum, and then decreases again to a minimum. If instead of

Newton's law we insert a somewhat different law of attraction

into the calculation, we find that, according to this new law,

the motion would still take place in such a manner that the

distance sun-planet exhibits periodic variations; but in this

case the angle described by the line joining sun and planet

during such a period (from perihelion-closest proximity to the

sun-to perihelion) would differ from 360°.