On August 21, 2017, fourteen states will witness one of the universe’s most impressive phenomenons — a total solar eclipse. (See the path across the states at http://www.astronomy.com/great-american-eclipse-2017.) Throughout human history, eclipses have brought fear and wonder into people’s lives. Fortunately, we now understand exactly what’s happening. But in the past, they have literally changed lives.
The May 2017 issue of Astronomy magazine contains a three-page article entitled “A Short History of Eclipses”. Here are two highlights from that article —
— According to the ancient Greek historian Herodotus, an eclipse stopped a war. In 585 B.C., the Lydians and the Medes were in the middle of a border conflict that had been dragging on for over five years. On May 28 of that year, they were fighting a battle along the Halys River in what’s now central Turkey when the sky suddenly began to darken. Everyone watched as the Sun disappeared. (The Greek philosopher Thales of Miletus had predicted the eclipse, but apparently no one told the warring armies.) This celestial event was taken as an omen to stop the war. Further, the daughter of the king of Lydia married the son of the king of the Medes to seal the deal. All that from a few moments of darkness during the day.
— In 1915, during World War I, Albert Einstein had published his revolutionary paper on his theory of general relativity. This theory was so radical that the press at the time claimed only three people in the world understood it. How to prove it?
Part of Einstein’s theory predicted light would be bent in a strong gravitational field, a phenomenon called gravitational lensing. In 1917, Sir Frank Watson, the astronomer royal of England, realized a total solar eclipse predicted for May 29, 1919 would be an opportunity to confirm Einstein’s idea.
Fortunately, World War I ended in November 1918, and the astronomers set to work. During totality, when the Sun was completely blocked, it would be in the constellation Taurus, near the stars of the Hyades cluster. In February 1919, English physicist Sir Arthur Eddington photographed the Hyades and made precise measurements of each star’s position. The month of May saw Eddington on the island of Principe off the west coast of Africa with a team of astronomers. During the eclipse, they photographed the stars nearest the Sun, then sailed home to compare those photos with the ones made in February. The comparison showed a definite deflection of the stars’ light, proving gravitational lensing and making Einstein an international celebrity.
There will be twelve total solar eclipses in the next twenty years. After August 21, the only one to cross a portion of the contiguous 48 states will be April 8, 2024. I hope you get to see one of them.