A Memorable Solar Eclipse
May 29th 2008
On this date (May 29) in 1919, Arthur Stanley Eddington and his crew traveled to the island of Príncipe off the coast of Africa to observe a total solar eclipse. Solar eclipses, which occur when the Moon’s shadow falls on Earth, aren’t all that rare, but this one was important: it was the first total eclipse after the First World War, and the first after Albert Einstein predicted the bending of light by gravity. In other words, the gravity of the Sun (and indeed any massive object) will affect the path that light takes from distant stars. As one of the most significant predictions of Einstein’s general theory of relativity (first published in 1916), the bending of light due to gravity needed precise measurements to prove it was happening.
Usually, though, the Sun’s light overwhelms the light from stars, so the effect of gravity is not detectable under ordinary circumstances—which is why Eddington needed to make his observations during a solar eclipse, when the Moon blocks much of the Sun’s light from reaching Earth. The effect of gravity showed up by comparing the position of stars during the eclipse to their position when the Sun is in a different part of the sky—the bending of light by the Sun makes the stars appear in different places. (Sizes and positions are exaggerated for clarity!)
For an object as small as our Sun (in cosmic terms, at least!), the bending of light is pretty small. However, when the object doing the bending is a galaxy, the results can be very dramatic: the image can be magnified, split into multiple images, and distorted. Called gravitational lensing, astronomers use the effect to map the mass distribution inside galaxies and to see much farther and fainter objects than would be possible otherwise. Here is a famous example, the “Einstein cross” quasar:
The four arms of the cross are separate images of the same distant galaxy!
So the solar eclipse observation of 1919 was not only the first verification of Einstein’s general theory of relativity, it was a hint toward the important discoveries to come.
