How The Last Great American Eclipse Almost Shocked Einstein
“While modern technology like the Hubble Space Telescope has revealed hundreds of these gravitational lenses, nothing comparable existed in the early 20th century. Instead, a clever substitute was concocted by astrophysicists of the time: since the positions of stars were very well-known, you could observe stars close to the Sun during the day, where the Sun’s gravity would pull on those light rays. Since Einstein’s theory and Newton’s theory gave differing predictions for how much light would be bent by that extra gravitational force – with Einstein predicting double the Newtonian amount – simply comparing observations during the day with ones taken of those same stars at night would allow you to prove whether Einstein or Newton was right.”
Next year, on August 21st, 2017, millions of Americans will delight in the first total solar eclipse to go from coast-to-coast in 99 years. The path of totality will range from Oregon to South Carolina, giving more than two minutes of the Sun being blocked by the Moon’s shadow to all along its central path. During this time, not only the Sun’s corona but also the background stars, never visible otherwise during the day, will be revealed. This isn’t merely a spectacular sight, but provides an opportunity to test Einstein’s general relativity directly: by measuring the deflection of starlight caused by the gravitational presence of the Sun! Although this was famously (and successfully) undertaken in 1919, the United States had a chance to beat the rest of the world in 1918, and almost succeeded!