On Aug. 21, sky-gazers from around the world will converge in the United States as a total solar eclipse charts a path from Oregon to South Carolina. In between, on Casper Mountain in Wyoming, you’ll find Don Bruns with his telescope.

A retired physicist, Bruns is using the rare opportunity to test Albert Einstein’s general relativity like Sir Arthur Eddington, who was the first scientist to test the theory back in 1919. At that time, Newton’s law of universal gravity was still vogue, but