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(N.B.-One complete revolution corresponds to the angle 2π in the

absolute angular measure customary in physics, and the above

expression given the amount by which the radius sun-planet

exceeds this angle during the interval between one perihelion and

the next.) In this expression a represents the major semi-axis of

the ellipse, e its eccentricity, c the velocity of light, and T

the period of revolution of the planet. Our result may also be

stated as follows: According to the general theory of relativity,

the major axis of the ellipse rotates round the sun in the same

sense as the orbital motion of the planet. Theory requires that

this rotation should amount to 43 seconds of arc per century for

the planet Mercury, but for the other Planets of our solar system

its magnitude should be so small that it would necessarily escape

detection.[26]