Eddington organized two expeditions to the southern hemisphere to record effects on starlight during a total solar eclipse. At totality the Sun’s rays would be completely occluded for several minutes, giving an observer enough time to examine the Sun’s warping of the space around it by measuring the bending of light rays from stars near the edge of the Sun’s disk on their way to Earth. Eddington himself led one of the teams down to the island of Principe off the coast of Africa. The other group, serving as backup, went to Sobral in Brazil. The backup plan proved most fortunate when Eddington’s voyage turned out to be literally a wash. A deluge of rain drenched Eddington and his team members as they tried to make out the stars through the clouds. They did manage to take some photos, but the ones from Brazil were generally much clearer. Merging these results, Eddington calculated the bending. It agreed reasonably well with general relativistic predictions.

A third test of general relativity, called gravitational redshift, involved the light emitted by the Sun itself. Resembling Doppler shifts, the predicted effect pertained to the reddening of light escaping a deep gravitational well. According to Einstein’s theory, the strong gravitational field near the surface of the Sun should slow down the rate of clocks there, resulting in the lowering of luminous frequencies. This postulate can be tested by looking at the spectral lines of atoms—which act like tiny timepieces. The same process should occur near the surfaces of other stars—especially compact ones like Sirius B, the white dwarf companion to Sirius A (the Dog Star). Unfortunately, while the Sun and other bright stars are easy to observe, the physics of their churning surfaces is hard to decipher. So this test was less clear-cut than either Mercury’s precession or the Sun’s light bending. Sirius B’s redshift would not be measured until the mid-1920s.

However, by 1919 the weight of the data for the other two tests was clearly in favor of Einstein’s theory. Eddington, who at that time was one of the few people in the world who properly understood it,