b'4 Carnegie Science|Summer 2019Chasing Einsteins Eclipse100 Years AgoBy Shaun HardyCarnegie had eclipse stations along the path of the 1919 eclipse (from left to right): Sobral, Brazil; Puerto Deseado, Argentina; Campo, Cameroon; and Cape Palmas, Liberia. Image courtesy Roberto Molar Candanosa, Carnegie Institution for ScienceL ouis Bauer was an eclipse veteran. In May 1900, fourLouis Bauer, founder of Carnegies Department of Terrestrial Magnetism (DTM), is en route to Liberia, April 1919 (top left). In May that year, he met with faculty and local dignitaries at Cuttington College, Liberia (top right). H. F. Johnston is observing years before he founded Carnegies Department ofmagnetic variation at the DTM eclipse station in Cape Palmas, Liberia, in May 1919 (bottom left). Andrew Thomson (bottom right, front) is with a DTM atmospheric-Terrestrial Magnetism (DTM), he organized the firstelectricity apparatus at Sobral, Brazil, also in May that year.systematic observations aimed at detectingImages courtesy Carnegie Institution for Science, Department of Terrestrial Magnetism Archivesgeomagnetic effects of a solar eclipse. Increasingly ambitious campaigns followed. For the transcontinental eclipse ofTwenty-two hundred miles to the west, their counterparts Daniel M. June 8, 1918, Bauer orchestrated a coast-to-coast observingWise and Andrew Thomson arrived in Sobral, 45 days after leaving program, stationing Carnegie teams from Washington State to theNew York on the steamer Hollandia.Gulf Coast. Bauer witnessed the spectacle from 11,800 feet up in theWise had joined DTM in 1913 as an observer and gained Rocky Mountains. Words will inadequately describe the mingledvaluable field experience over the next six years. He was an apt feelings one has when the supreme moment arrives for whichchoice to lead DTMs work in Brazil. more or less extensive preparations have been made . . . There wasMay 29 dawned cloudy in Sobral, but skies cleared as the a feeling almost of exhilaration, he wrote about totality in Samoa ineclipse commenced. Thomson recorded changes in the electrical 1911. properties of the atmosphere under a makeshift shelter. Ironically, The end of WWI meant the reopening of international travelWise, manning magnetometers in the basement of a nearby house, and renewed research opportunities. The eclipse of May 29, 1919,may not have seen the eclipse at all. A short distance away, was eagerly anticipated as a test for Einsteins 1916 theory ofastronomers from Greenwich Observatory photographed the first general relativity. British astronomers would travel to Brazil anddirect evidence for the Einstein effect. Principe Island off the coast of West Africa in search of theRacing east across the Atlantic at 1,500 miles per hour, the predicted gravitational bending of starlight passing near the Sun.Moons shadow reached the DTM team at Cape Palmas an hour and Bauer saw another opportunity to search for minute, temporarya half later. Totality there lasted more than six minutes and good changes in the geomagnetic field as the Moons shadow sweptgeomagnetic data were obtained, but plans to observe atmospheric-across the Earth. electricity were thwarted by degraded batteries and no Hedging his bets against problems, Bauer selected observing sitesreplacements were found.on two continents. Cape Palmas, Liberia, and Sobral, Brazil, would beAfter the eclipse Bauer returned to England to report his results Carnegies principal eclipse stations. Observers positioned outside theto the Royal Astronomical Society and then to Brussels where he path of totality, in Peru, Argentina, Cameroon, and Washington, D.C.,helped establish the International Union of Geodesy and Geophysics. would also collect data. Cooperative arrangements were made withAfter the 1919 expeditions Bauer concluded, There cannot be magnetic observatories in a dozen countries to obtain simultaneousany doubt that the Earths magnetic field is subject to an observations. Bauers experiment would be global. appreciable magnetic variation during a solar eclipse. Another Bauer and chief assistant H. F. Johnston reached Cape Palmasfourteen years would pass before British geophysicist Sydney on May 5 after a three-week voyage from England. They unpackedChapman provided a theoretical framework for understanding this instruments for the eclipse and the magnetic survey work nearby.elusive phenomenon. '