In 1915, Albert Einstein proposed the theory that gravity could influence light to bend, something that has come to be known as gravitational lensing. His theory was that the gravity from mass (e.g., a star or black hole) would pull on light photons as they traveled by, moving them out of their original trajectories. Of course, at the time, the sun was the only thing with enough mass to bend light that was close enough for scientists to observe. Thus, to test the theory, scientists needed to compare the observed position of the stars in the presence versus the absence of the sun. During the night, there would be no gravitational lensing, as the sun was not present. Conversely, during the day, the sun would theoretically bend the same light. Einstein's theory would be correct only if the position of the stars near the sun during the day appeared different than at night.

Though many people believed Einstein's theory was correct, there was one problem: no one could test it. During the day, the sun was so bright that no one could see the stars, let alone gauge whether their position appeared to shift as a result of gravitational lensing. No one could test the theory, that is, until Sir Arthur Eddington took advantage of a total solar eclipse. As fate would have it, when the moon passed between the sun and the earth in 1919, it blocked out the sun's rays such that Eddington could actually see the stars immediately around the sun.