When it comes to matters of time and space here on Earth, we are as we see each other. Clocks around the globe sync to the picosecond, and our shared measures of distance are just as precise. These things, we might conclude, are immutable. And, as such, the universe too is as we see each other—a rigid expanse ruled by a master tape measure and a divine clock. Or what?
In December 1919, the six foot-tall astronomer Sir Arthur Eddington stood before a lecture hall at Trinity College, Cambridge and explained that he was really only three-feet tall. There, at Cambridge on planet Earth, he was most assuredly six-foot, but this was only a provisional measure. We could even say it's a relative measure. If Eddington were to be traveling at a rate of, say, 161,000 miles per second, he explained, he would only be three feet tall. But still the same Eddington.
Several months earlier, Eddington was on the tiny island of Príncipe, off the coast of West Africa. His expedition was one of two launched in May of 1919 to observe a total solar eclipse; the other was positioned off the coast of Brazil. Both locales were chosen for being directly under the Sun's track across the sky on the day in question and for having generally good weather.
The goal of the two Royal Astronomical Society-funded expeditions was to observe starlight passing near the blacked-out Sun. As the Sun is really the only thing in our local neighborhood with sufficient mass to warp space-time to the point of being observable to 1919 astronomers, starlight traveling from some distant source to Earth that happened to pass near the Sun should be deflected as a consequence of the warped space-time predicted by Einstein.
That was the whole point: acquiring direct evidence to support Einstein's General Theory of Relativity. At the time, said theory wasn't quite the slam-dunk we take it as today.
"The days preceding the eclipse were very cloudy," Eddington wrote in a paper published in November, 1919. "On the morning of May 29, there was a very heavy thunderstorm from about 10:00 AM to 11:30 AM. The Sun appeared for a few minutes, but the clouds gathered again. About a half hour before totality, the crescent Sun could be glimpsed occasionally, and by 1.55 it could be seen continuously through drifting cloud."
The Príncipe expedition managed to capture 16 photographic plates of the eclipse and surrounding stars. All but the last few were obscured by clouds.
The images from the second, Brazil-based expedition were better, and, together, the expeditions offered a clear demonstration of Einstein's prediction. The Sun had indeed deflected passing light with a strength inversely proportional to the passing light's distance from the Sun's center—that is, in accordance with the strength of gravity.
The speed of light is constant, according to Einstein's Theory of Special Relativity, and so deflections in its passage can mean only that the very route through space-time it travels through must be changing. Space-time warps in the presence of gravity, and this changes everything. Quoth the three-foot astronomer: "Every body continues in its state of rest or uniform motion in a straight line, except insofar as it doesn't."