MARGINALIA
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 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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