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Judging Einstein

Before most physicists would believe the claims of relativity, they required proof—which would come in the form of a solar eclipse

J. Donald Fernie

Partly Cloudy

Around the same time, an eclipse in the United States in June 1918 was almost entirely obscured by clouds, but Campbell's team did get some photographs. These poorly exposed plates seemed to indicate no relativistic effects, much to the delight of Einstein's skeptics, including Campbell.

The original caption for the graphical explanation...Click to Enlarge Image

The eclipse of May 29, 1919, was to start near the border between Chile and Peru, then traverse South America, cross the Atlantic Ocean and arc down through central Africa. No part of the path was far from the equator, and the desirable, longest-lasting portion was in the Atlantic, a few hundred miles from the coast of Liberia. The British planners decided that the tiny island of Principe, nestled in the crook of Africa's Gulf of Guinea, would be best despite the poor astronomical viewing from low-lying tropical regions. The choice of Principe introduced other challenges. One modern travel agency advises prospective visitors to the island that "It's best to go between June and September. The rest of the year is muggy and hot—you'll be swimming in rain and your own sweat." Just in case Principe was cloudy at the crucial time, the British sent a second expedition to observe the eclipse from Sobral, in eastern Brazil.

The main instruments at both sites were existing astrographic telescopes of 33-centimeter aperture designed specifically for photographing star positions with high precision. Although these telescopes were designed to automatically follow the stars, their temporary emplacement in the field required each telescope to be immobilized as a clockwork-driven flat mirror tracked across the sky and fed light to the main lens. As an afterthought, the Brazil contingent added a small 10-centimeter telescope to its roster. In the end, it saved the day.

The expeditionaries set out months ahead of the eclipse to allow for travel difficulties. Although the war officially ended in November 1918, chaos continued for months thereafter. Upon arrival, they had to evaluate the terrain, choose a site, and set up and test their equipment. Eddington's group arrived at Principe in late April and, amid the heat and rain, found themselves under such constant attack by biting insects that they needed to work under mosquito netting most of the time. The rain grew worse as May advanced, and the day of the eclipse began with a tremendous storm. The rain stopped as the day wore on, but the totality phase of the eclipse would start at 2:15 p.m. and last only five minutes. Eddington wrote:

About 1.30 when the partial phase was well advanced, we began to get glimpses of the Sun, at 1.55 we could see the crescent (through the cloud) almost continuously and large patches of clear sky appearing. We had to carry out our programme of photographs in faith. I did not see the eclipse, being too busy changing plates, except for one glance to make sure it had begun.... We took 16 photographs ... but the cloud has interfered very much with the star images.

The weather in Brazil was much better—beautifully clear, in fact. The observers took 19 photos with the astrograph and eight with the small telescope. But when the photographs were developed, they found that despite their precautions, the astrograph's pictures showed, according to Dyson, "a serious change of focus, so that, while the stars were shown, the definition was spoiled." Even under ideal conditions, the predicted relativistic displacement on the photographs was only 1/60 of a millimeter—about a quarter of the diameter of a star on a sharply exposed image. Although they could measure such a minute shift, the poor focus made this task nearly impossible. By contrast, the small telescope's photographs were clear and sharp, but on a reduced scale.

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