Famous Eclipse of 1919

The 1919 solar eclipse provided the ideal opportunity to test the theory of General Relativity of Einstein as the eclipsed sun would be close to the Hyades star cluster amongst a group of relatively bright stars. During the eclipse rays of light from stars, passing close to the sun would be bent so that the star images seen from the earth would seem to move away from the sun. The effect for a ray of light grazing the sun is 1.75 seconds of arc which is twice as large as the shift predicted by Newton 's theory. This is the same phenomenon as gravitational lensing, where a cluster of galaxies can produce multiple magnified images of a galaxy twice as far away, which has been seen using the Hubble Space Telescope.

Two expeditions were organized, one to Sobral in Brazil and the other including Sir Arthur Eddington, to the island of Principe off the west coast of Africa. At Principe there was a thunderstorm in the morning which cleared in time to take photographs, only two of which showed star images. At Sobral the clouds also cleared in time to make observations and 7 usable plates were obtained.

	The shifts were typically about one fifth the diameter of the star images and difficult to measure accurately. However, in November 1919 Sir Frank Dyson announced to a joint meeting of the Royal Society and the Royal Astronomical Society, that the results confirmed Einstein's theory.