the sense of the orbital motion. The value obtained for this rotary movement of the orbital ellipse was seconds of arc per century, an amount ensured to be correct to within a few seconds of arc. This effect can be explained by means of classical mechanics
only on the assumption of hypotheses which have little probability, and which were devised solely for this purpose.
On the basis of the general theory of relativity, it is found that the ellipse of every planet round the sun must necessarily rotate in the manner indicated above; that for all the planets, with the exception of Mercury, this rotation is too small to be detected with the delicacy of observation possible at the present time; but that in the case of Mercury it must amount to seconds of arc per century, a result which is strictly in agreement with observation.
Apart from this one, it has hitherto been possible to make only two deductions from the theory which admit of being tested by observation, to wit, the curvature
of light rays by the gravitational field of the sun, Observed by Eddington and others in 1919. (Cf. [appendix:III]Appendix III.)
and a displacement of the spectral lines of light reaching
us from large stars, as compared with the corresponding lines for light produced in an analogous manner terrestrially (i.e. by the same kind of molecule). I do not doubt that these deductions from the theory will be confirmed also.
IIIConsiderations on the Universe as a WholeConsiderations on the Universe