The position of the perihelion of Mercury
changes with time, the axis of the orbit slowly rotates. (see diagram below). Originally this was thought to be due
to the combined gravitational pull of all the other planets but this predicted value was found to be wrong by
about 43" per century. The difference is explained by the general theory of relativity. This divergence from
theory was then explained by the existence of a gravitational attraction of a then undiscovered planet
called Vulcan between Mercury and the Sun.
The true explanation was given by the General
theory of Relativity.
