Expeditions That “Discovered” the Sun

Trudy E. Bell, Science writer and historian of astronomy

A scientific expedition of Princetonians photographs a solar eclipse in Wadesboro, North Carolina. Seeley G. Mudd Manuscript Library, Princeton University ArchivesThe first observation of a solar eclipse was a classic case of serendipity. On July 8, 1842, astronomers traveled to southern France and northern Italy to time the passage of the moon across the face of the sun—a rare opportunity to refine knowledge of the moon’s orbit as well as latitudes and longitudes of European maps. As the black moon finally totally eclipsed the sun, however, they were stunned to see its circumference dotted with brilliant ruby flames in contorted loops. From then on, astronomers sought to chase every total solar eclipse to learn more about the physical nature of the sun.

Victorian eclipse expeditions were arduous adventures. Lugging tons of equipment, dozens of support staff, and even wives and children on mules or in dugout canoes, astronomers arrived several months before the day of the eclipse to build temporary observatories and photographic darkrooms. For weeks, they painstakingly determined their latitude and longitude, rehearsed observing protocols, and monitored local meteorology—praying for clear skies. Sometimes, despite all cost, effort, and preparation, the expedition’s crucial moment was spoiled by clouds or illness. Between 1842 and 1919, expeditionary astronomers collectively observed just a single hour of totality—but major discoveries included identifying the ruby loops as prominences and the pearly halo as the sun’s tenuous outer atmosphere (outer corona), finding the element helium before locating it on Earth, and measuring how starlight bends in the sun’s gravitational field, a key prediction of Einstein’s general theory of relativity.

Observers of the total Solar Eclipse Expedition in Spain, 1860
Source: “The Balkan Lecture: On the Total Eclipse of July 18th, 1860, observed at Rivabellosa, near Miranda de Ebro, in Spain,” Warren De La Rue. Philosophical Transactions of the Royal Society of London, Volume 152, Pages 363 (left) and 336 (right); cited in Trudy E. Bell, “The Eclipse Chasers” Griffith Observer (June 2001)

The 1842 images are colored drawings by Wilhelm von Biela, showing the “red protuberances” (prominences) and corona that so astonished astronomers. Trudy E. Bell, “The Victorian Space Program,” The Bent (Spring 2003)

A close-up of the interior of the Des Moines temporary observatory shows the equipment that captured more than one hundred wet collodion photographs of the sun’s outer corona for the first time in 1869. Mounted at the eye end was a camera box for holding the dripping photographic plates, onto which the sun’s image was projected. The observer is in the act of timing an exposure with a chronometer (strapped to the tripod). On the wall behind the telescope and chronometer are the small doors of the dumbwaiters used for communicating with those inside the darkroom; the observer received and passed wet photographic plates for exposure and developing through the dumbwaiters." Source: “Report of Dr. Edward Curtis, U.S.A.,” Appendix II, Reports on the Observations of the Total Eclipse of the Sun, August 7, 1869, U.S. Naval Observatory, p. 126; cited in Trudy E. Bell, “The Victorian Space Program,” The Bent (Spring 2003)

Temporary observatories for total solar eclipse expeditions—such as this wooden building, erected in Des Moines, Iowa, for the eclipse of August 7, 1869—were often elaborate. This building was used by U.S. Army Assistant Surgeon Brevet Major Edward Curtis, an officer skilled in photography, and his assistants. A southern room under the retractable roof sheltered several astronomical telescopes (note the observers and instruments); the northern quarter (invisible behind the partition and under the wooden roof) was a ventilated photographic darkroom complete with indoor plumbing for processing wet collodion plates. Source: Plate I from “Report of Dr. Edward Curtis, U.S.A.,” Appendix II. Reports on the Observations of the Total Eclipse of the Sun, August 7, 1869, U.S. Naval Observatory; cited in Trudy E. Bell, “The Victorian Space Program,” The Bent (Spring 2003)

J. Norman Lockyer, the British astronomer who discovered helium in the sun, traveled to India for the December 12, 1871 total solar eclipse. The corona still puzzled astronomers at that time. Lockyer was convinced that the corona is the outermost atmosphere of the sun, but detailed understanding of its true nature still eluded him at this time. In this illustration, Lockyer, seated by his telescope, is shaded by a local performing parasol duty. Lockyer chose the top of the tower at the old fort at Bekul for his eclipse post. Source: Bryan Brewer,Eclipse (Seattle: Earth View, 1978); cited in Trudi E. Bell, “The Eclipse Chasers,” Griffith Observer (June 2001)

The pneumatic commutator and its battery of instruments were invented by Amherst College Observatory Director David Peck Todd and mounted at Cape Ledo, Angola, for the total solar eclipse of December 22, 1889. In all, twenty-three telescopes (principally photographic) were attached to a massive clock-driven polar axis and pointed toward the sun, parallel to each other. Exhaust air currents through pneumatic tubes, connected with each telescope and photographic plate-holder, were controlled by a slowly moving perforated sheet of paper, similar to the music sheets used in automatic organs or player pianos. The machine exposed more than three hundred photographic plates. Source: Stars and Telescopes by David P. Todd, (Boston: Little, Brown & Co., 1899), page 361; cited in Trudy E. Bell, “The Victorian Space Program,” The Bent (Spring 2003)

Todd’s electric Amherst machine, shown at center in its temporary observatory building with the hinged roof open, shot more than four hundred photographs in the two minutes and forty seconds of totality during the August 9, 1896, eclipse in Esahi, Japan. Endless chains of photographic plates of different sizes passed through the focal plane of each of its twenty telescopes at varying rates of speed. The exposure times for all twenty instruments were controlled by a slowly revolving copper cylinder full of pins; each pin, as it passed along, touched its appropriate metal tooth, closing an electrical circuit that set in motion some particular instrument at a prearranged instant. The entire framework carrying all twenty instruments was driven by an hourglass-like glycerine clock to follow the sun.Source: Mabel Loomis Todd, Plate Corona and Coronet (Boston and New York: Houghton, Mifflin, & Co., 1899); cited in Trudy E. Bell, “The Victorian Space Program,” The Bent (Spring 2003)

The longest focal-length photo heliograph used for photographing the inner corona during a nineteenth-century eclipse expedition was that used by Samuel Pierpont Langley of the Smithsonian Astrophysical Observatory during the total solar eclipse of May 28, 1900, which he observed from Wadesboro, North Carolina. Source: cited in Trudy E. Bell, “The Victorian Space Program,” The Bent (Spring 2003)