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 . The line of the orbit would not then be a closed one
but in the course of time it would fill up an annular part of the
orbital plane, viz. between the circle of least and the circle of
greatest distance of the planet from the sun.
According also to the general theory of relativity, which differs of
course from the theory of Newton, a small variation from the
Newton-Kepler motion of a planet in its orbit should take place, and in
such away, that the angle described by the radius sun-planet between
one perhelion and the next should exceed that corresponding to one
complete revolution by an amount given by
image051
(_N.B._ One complete revolution corresponds to the angle 2 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]
[26] Especially since the next planet Venus has an orbit that is
almost an exact circle, which makes it more difficult to locate the
perihelion with precision.
In point of fact, astronomers have found that the theory of Newton does
not suffice to calculate the observed motion of Mercury with an
exactness corresponding to that of the delicacy of observation
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