IN 1915 Prof. Einstein predicted, as a consequence of the generalised theory of relativity, that a ray of light from a star would be bent in its passage through the sun's gravitational field. The amount of this deflection he gave as 1.75″(a/r), where a is the sun's radius, and r the nearest distance of the ray to the sun's centre. As a ray of light reaches us in the direction of the tangent to its path, the apparent position of a star, photographed during an eclipse of the sun, should therefore be displaced by an angle 1.75″(a/r) outwards from the sun's centre. The field of stars surrounding the sun should thus be distorted in a definite manner by an amount within the range of accurate astronomical observation. In Fig. 1, if a, b, c are the positions of the stars as seen at ordinary times, and if S is the centre of the sun during an eclipse, then the positions in which the stars are seen or photographed during an eclipse will be A, B, C. If a were at a distance 30′ from the sun's centre the displacement aA would be 0.87″, and if? were at a distance 90′ from the sun's centre its displacement would be 0.27″. These are, roughly speaking, the greatest and least displacements which can be obtained in practice. Nearer than 30′ from the sun's centre a star's image is liable to be drowned in the corona. At greater distances than 90′ good images can scarcely be obtained with a simple object glass, while a doublet or other combination introduces some difficulties.
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DYSON, F. Relativity and the Eclipse Observations of May, 1919. Nature 106, 786–787 (1921). https://doi.org/10.1038/106786a0