When starlight passes near a massive object like the Sun, its path is bent making the source appear to be in a slightly different position than it should be. This effect was first observed during a Solar eclipse in 1919 by Sir Arthur Eddington. Eddington observed a pair of stars whose separation was well known during a solar eclipse when one of the stars was lensed by the Sun. The lensed star’s position (star A in Figure is closer to the Sun), was slightly shifted and the separation between the two stars appeared to be smaller than previously measured.
Eddington’s solar eclipse expedition confirmed General Relativity and made Einstein a celebrity, overnight. It turned out that the experiment was not accurate enough – the uncertainty in the measurement was larger than the result. A similar solar eclipse experiment that was done in the 1967 and it did in fact confirm general relativity.
Today, with advancements in telescope and camera technology we see many examples of gravitational lensing. You saw a number of lensed galaxies or supernovae in a previous section.