The Great Eclipse Of 1919

Eddington knew a few things about eclipses (he had gone eclipse chasing to Brazil in 1912 as a member of a large British party which had been clouded out), and he saw how a total solar eclipse could provide a unique opportunity to provide verification for Einstein's theory. The reason for this is illustrated in Figure 4-1.

Consider the light from some distant star passing by the Sun. The path of the light is bent by the Sun's gravity (the rule you may have been taught at school that light travels in straight lines is only a first-order approximation). According to Einstein's theory the bending of the path of the light beam is twice that which Newton's theory of gravity would suggest.

In principle this provides a test, but when one does the sums it turns out that the angles are extremely small. Even for light passing just above the Sun's surface, for which the bending is greatest, the direction change is less than two seconds of arc. How much is that? A degree may be split into 60 minutes of arc, each of

FIGURE 4-1. The deviation of starlight produced by the mass of the Sun, detectable during a total solar eclipse. The paths the light takes from the distant stars at left follow the heavy lines, but from Earth the arrival directions extrapolated backwards appear further from the Sun, as shown by the faint lines. The deflection angles are shown greatly exaggerated. Einstein's relativity theory said that the deflection would be twice that based on Newtonian gravitational theory, and this was verified using the great eclipse of 1919.

FIGURE 4-1. The deviation of starlight produced by the mass of the Sun, detectable during a total solar eclipse. The paths the light takes from the distant stars at left follow the heavy lines, but from Earth the arrival directions extrapolated backwards appear further from the Sun, as shown by the faint lines. The deflection angles are shown greatly exaggerated. Einstein's relativity theory said that the deflection would be twice that based on Newtonian gravitational theory, and this was verified using the great eclipse of 1919.

which comprises 60 seconds of arc (using the addendum "of arc" to show that we are referring to angles here, not units of time). To put that into some context, two seconds of arc is the apparent width of a matchstick viewed from 220 yards, almost twice the length of a football field. The test would involve being able to differentiate between a single matchstick width, and merely half that as the Newtonian theory would have it.

The problem is that starlight passing so close by the Sun is drowned in the solar glare at all times except during a total eclipse, and so Eddington proposed making observations during such an event. Just any eclipse would not do though. Not only did Eddington need totality, he also needed stars, because the project would not work unless there were several bright stars close to the limb of the Sun during the eclipse. Looking up the eclipse predictions, Eddington saw that one of those represented in Figure 2-2, occurring on May 29, 1919, allowed a unique opportunity. Not only was the totality long, at 6 minutes and 51 seconds, but it was also in late May when the Sun is passing through the constellation Taurus, and crossing a rich cluster of bright stars known as the Hyades.

His mentor, the Astronomer Royal Sir Frank Dyson, was so enthused about the concept that he lobbied the government to avoid having the youthful Eddington drafted to fight in the First World War. Instead Eddington was allowed to prepare for the great eclipse expedition of 1919. The British foray was in several parts, with Eddington leading one group to Principe (a tiny island owned by Portugal, just north of the equator and 150 miles from the African coast), while another headed for the opposite side of the Atlantic, setting up their equipment at Sobral in northeastern Brazil. A contemporary map of the eclipse track, indicating when the footprint reached different locations, is shown in Figure 4-2.

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