The coast-to-coast eclipse that disappointed and delighted

The total eclipse that will travel from Oregon to South Carolina on 21 August is a once-in-a-lifetime event. Although solar eclipses occur fairly regularly—the path of totality of 71 eclipses touched at least part of the US during the 20th century—ones that sweep across the entire continental US are far rarer. The last coast-to-coast total eclipse in the US occurred just over 99 years ago on 8 June 1918 . The story of the 1918 eclipse highlights such events’ long-standing importance for science and reminds us that weather can be a capricious obstacle for astronomers and amateurs alike.

Preparations for eclipse observations began well before the big day. “Astronomers now getting ready for 1918 Sun eclipse,” the Hartford Courant proclaimed on 5 November 1917. The total eclipse would make landfall in Oregon around 4:00pm local time and would be visible in all 48 contiguous states before leaving the continental US off the Florida coast just before sunset.

One of astronomers’ fondest hopes for the 1918 eclipse was that they would be able to test a key prediction of Albert Einstein’s general theory of relativity. Einstein had predicted that a massive body like the Sun would curve spacetime around itself. If Einstein was right, then starlight passing near the Sun would be deflected about twice as much as predicted by Newtonian physics. A total eclipse would render this extra shift in the stars’ apparent positions visible to astronomers’ cameras. However, Scientific American warned readers that during the 1918 eclipse, the Sun would be “in a field poor in stars,” making it somewhat unlikely that astronomers would be able to test Einstein’s theory.

Astronomers also hoped to record new information about the Sun’s corona. The Courant reported that scientists were particularly excited that the eclipse’s path would include the Denver area, where high altitudes seemed favorable to clear observation. Astronomers at Denver University (now the University of Denver) planned to collaborate with a delegation from the Yerkes Observatory at the University of Chicago to photograph the corona. Science reported that the Yerkes team would bring its spectroscope to Denver to capture the spectrum of the corona; the researchers also hoped to measure the corona’s speed of rotation. Meanwhile, the US Naval Observatory planned an expedition to Baker City, Oregon, where totality would last the longest.

The weather threw a wrench into astronomers’ plans when the day dawned cloudy across much of the country. S. A. Mitchell, the director of the University of Virginia’s Leander McCormick Observatory, was in Baker City for the eclipse. “Soon after 3 o’clock the clouds became denser,” he wrote in the New York Times, “and so black that the whole sun was blotted out. The hearts of the assembled astronomers sank.”

Baker City had a relatively happy ending: By 3:45, only thin clouds obscured the view of the corona. “What was lacking in the outer corona was more than made up by the brilliant inner corona,” Mitchell enthused, pronouncing the 1918 eclipse the “most brilliant” he had ever seen. Although observers did succeed at taking some photographs of the corona, the 1918 eclipse’s most famous image is a painting (top) by Howard Russell Butler, who took on the challenge of rapidly drawing the corona and recording its colors during the two minutes of totality.

Observers in Denver were not quite so lucky. Astronomers “found that their elaborate preparations had gone for naught,” lamented the Times, as “heavy clouds covered the sky during the greater part of the eclipse.” Similarly, clouds and a light scattering of rain obscured the eclipse almost completely in Jackson, Mississippi, disappointing southern astronomers who had gathered at Millsaps College’s James Observatory. And when the photographs were developed, it became clear that no one in the US had been able to obtain good images of starlight near the Sun. A true test of Einstein’s theory would have to wait for the 1919 total eclipse off the west coast of Africa (see the article by Daniel Kennefick, Physics Today, March 2009, page 37 ).

Weather conditions were more favorable in the parts of the country that witnessed only a partial eclipse. Skies were clear in New York City, where enterprising young newsboys “did a thriving business renting pieces of smoked glass at ‘a nickel a look’” to the assembled crowd in Times Square, according to the Times. Observers in Boston, Knoxville, south Florida, and Detroit also reported clear views and large crowds. “Some few ventured to observe the eclipse with the naked eye—once,” the Detroit Free Press reported the next morning. “They did not try a second time.”