This photo made Einstein the national Icon he was, and still is today. But why? What's so special about a solar eclipse? There are many of them that happen all the time.
In the early 1900's, Albert Einstein proposed that Newtons idea of gravity, first proposed in his Principia Mathmatica published in 1687, was incorrect. That would mean that for over 200 years the world didn't quite understand the true meaning of gravity and its role in space. Newton's law states that "every point mass attracts every other point mass by a force acting along the line intersecting the two points. The force is proportional to the product of the two masses, and inversely proportional to the square of the distance between them." In other words:
This would mean that gravity is universal and constant. Einstein saw a flaw in the theory, in that, he believed gravity wasn't the attraction between objects, but instead, the influence of objects on space itself, bending space like a cloth. In 1919, he set out to try and prove him wrong. If Einstein were right, then the same mathematics that prove his idea of bending space would also apply to the light of distant stars. Einstein proposed an ingenious and clever way of measuring bending light, by using a total solar eclipse.
If light were to bend around the sun, then all you would need to do is observe some bright stars in the location that they would be in the path of a TSE (total solar eclipse) and then observe it again when the TSE is "in the way". There would have been no way to measure the light around the sun without the eclipse, because the sun would drown out any starlight. So this was one of the only opportunities they had, at least in Einstein's lifetime, and they had to get it right.
In order to prove to the world Einstein was right, not only did they need picture proof, the world needed a prediction. Einstein calculated what he believed the displacement of starlight would be with his general theory of relativity, and Newtons predictions of starlight bending due to the attraction of objects - including starlight. As you can see by the graph, Newton predicted the displacement wouldn't be that much, and Einstein predicted, of course, that the closer the object came to the gravitational force of the sun, the more it warped around it, displacing the measurements from newtons results by nearly a factor of 2.
Einsteins work landed in the hands of Arthur Eddington (as seen above) and in the time they wanted to set out to take pictures, the world was already at war in WW1.
Despite the war, and despite Einstein and Eddington being on opposing sides - Eddington, along with Frank Dyson set out to test his theory. The May 1919 eclipse was ideal because it would be spanning across a large cluster of bright stars called the Hyades Star Cluster. This meant they could identify several bright stars, only proving the displacement further and without question. In 1917-1918 they set off to 2 different locations where the TSE would cross. One in Principe in west Africa and the other in Sobral, Brazil.
From there they took pictures of the Eclipse, and of course, Einsteins displacements were nearly perfect. At least, in comparison to newtons. It was enough to make Einstein Famous overnight and debunking a 200 year old theory of gravity. It's this pivotal critical thinking that made it possible for us as the human race to streamline spaceflight, discoveries, and many more mysteries - such as black holes and quantum mechanics. Without this discovery, we may very well may have been earthbound for hundreds of more years.