The GP is correct that Eddington's observations during the 1919 eclipse had to do with the deflection of starlight. General relativity predicted that starlight

The GP is correct that Eddington's observations during the 1919 eclipse had to do with the deflection of starlight. General relativity predicted that starlight passing by the Sun would be deflected by a certain amount, and Eddington's observations showed that general relativity's prediction was correct. The discrepancy between Mercury's perihelion precession and Newtonian gravity had been observed long before Einstein developed general relativity. Calculations under general relativity correctly determined Mercury's perihelion advance, but general relativity did not predict the perihelion advance, since it was already known.