The Sun had to disappear
General relativity describes gravity as curved spacetime. Light passing near the Sun should therefore bend, but ordinary sunlight hides the background stars. A total eclipse offered the brief necessary darkness.
Frank Dyson organized expeditions to Principe and Sobral in 1919. The war had just ended; British scientists testing a German physicist’s theory also carried symbolic weight.
Diagram description
Static physics diagram: eclipse_gravitational_lens.
Clouds, warped equipment, and tiny displacements
Principe was cloudy. One Sobral telescope lost focus in the heat. The surviving plates were compared with reference images taken when the Sun was elsewhere.
Einstein predicted about 1.75 arcseconds of deflection near the solar limb. The announced result supported it and made him internationally famous.
Later historians debated the difficult data and Eddington’s preferences, but no simple fraud story fits the records. Better measurements repeatedly confirmed gravitational bending. The eclipse mattered because a shift smaller than a hair seen from meters away tied an abstract geometry of spacetime to positions on a photographic plate.