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We must draw attention here to one of these deviations. According

to Newton's theory, a planet moves round the sun in an ellipse,

which would permanently maintain its position with respect to the

fixed stars, if we could disregard the motion of the fixed stars

themselves and the action of the other planets under

consideration. Thus, if we correct the observed motion of the

planets for these two influences, and if Newton's theory be

strictly correct, we ought to obtain for the orbit of the planet

an ellipse, which is fixed with reference to the fixed stars.

This deduction, which can be tested with great accuracy, has been

confirmed for all the planets save one, with the precision that

is capable of being obtained by the delicacy of observation

attainable at the present time. The sole exception is Mercury,

the planet which lies nearest the sun.