Today, a modern astronomer is just as likely to call herself an astrophysicist. But that has not always been the case, especially in the United States. Traditional “positional” astronomy (marked by the practical application of astronomical techniques, such as charting celestial motions for the purpose of navigation) prevailed in the US long before “physical astronomy” set the stage for probing the physical nature of stars and planets. Central to this change was the adoption of laboratory techniques that redefined the goals and methods of astronomical research.

But what sparked this transition in the US, where traditional positional astronomy long prevailed? Jennifer Lynn Bartlett and Thomas Hockey explore this pivotal moment in their newly published article in Physics in Perspective, “The Total Solar Eclipse of 1869 as Stimulus for Adoption of Physical-astronomy Techniques in the United States ” (open access).

On August 7, 1869, a total solar eclipse swept across the United States. For the general public, it was a rare celestial spectacle. But for American astronomers, it was a critical turning point—what Bartlett and Hockey convincingly argue was a moment of disciplinary transformation. The story they tell is both institutional and individual. It centers on how the US Nautical Almanac Office (NAO)—a government office with no telescopes, no mandate for physical observation, and no staff with formal spectroscopic expertise—mobilized a major eclipse expedition that would help launch astrophysics in the US.

American astronomy before 1869: practical and positional

Prior to the mid-nineteenth century, American astronomy was largely a tool of navigation. Observatories served maritime interests, and astronomical practice emphasized precision measurement: the calculation of celestial positions, the calibration of time, and the prediction of planetary motion. These were the purviews of institutions like the NAO, the US Naval Observatory (USNO), and the US Coast Survey (USCS), employing astronomers trained primarily in mathematics and mechanics, not experimental physics.

By contrast, European observatories were beginning to embrace new methods imported from the physical sciences. Spectroscopy allowed astronomers to identify the chemical composition of stars and the Sun through their light. In particular, the Heidelberg laboratories of Gustav Kirchhoff and Robert Bunsen played a foundational role: in 1859, they linked the dark lines seen in the solar spectrum with bright emission lines produced in chemical laboratories, enabling astronomers for the first time to determine the Sun’s composition.

A government expedition into unknown territory

Yet the total solar eclipse of 1869 offered an extraordinary opportunity to accelerate the adoption of these new methods. With totality set to cross much of the continental US, from Dakota Territory to the Carolinas, the eclipse was impossible for American astronomers to ignore. Multiple federal institutions mounted expeditions, but the NAO’s was the most ambitious—and surprising. As Bartlett and Hockey detail, NAO Superintendent John H. C. Coffin saw the eclipse as a chance to demonstrate that his agency could do more than calculate almanacs.

Coffin was rooted firmly in the traditions of positional astronomy. As Barlett and Hockey put it, he “literally wrote the textbook on navigation at sea” Navigation and Nautical Astronomy (1868). But with the eclipse, he could envision an expanded government role for astronomers. Though NAO staff lacked observational experience, Coffin assembled a team of academic collaborators, secured endorsements from scientific societies, and convinced Congress to appropriate $5,000 (over $100,000 today) for the endeavor.

Whether or not these organizations fully realized it, Coffin was just as interested in physical observations of the Sun during the eclipse as he was in traditional measurements of time and position. Coffin’s planning was logistically masterful and highly effective. His success in securing funding and resources was all the more remarkable given that the US Treasury was still recovering from the Civil War. He partnered with railroad companies to transport equipment and personnel, arranged to borrow telescopes and spectroscopes from universities, and coordinated with the USCS to find ideal observation stations. Most significantly, Coffin recruited Dartmouth physicist Charles A. Young, a rising expert in solar spectroscopy, to lead the scientific charge.

The central observing site was Burlington, Iowa, chosen for its location near the eclipse centerline and its likelihood of clear skies. Additional stations were established in Mount Pleasant and Ottumwa, Iowa. Each became a small laboratory in the field, attempting to deploy spectroscopy, photometry, and other physical techniques in what was, for many participants, their first experience with what we now call astrophysics.

Finding “coronium”

The most successful results came from Burlington, where Young observed through a state-of-the-art five-prism spectroscope mounted on a Merz & Söhne telescope. His observations were systematic and careful: he recorded the absorption lines of the solar photosphere, the bright emission lines of chromospheric prominences, and, most remarkably, a faint green emission line from the solar corona.

This green line was a mystery. It did not correspond to any known terrestrial element, leading Young (and later William Harkness, observing from a USNO station) to propose a new element: “coronium.” Though this hypothesis was eventually disproven—in 1942, the Swedish spectroscopist Bengt Edlén would identify the line as due to highly ionized iron—Young’s 1869 observation was a foundational moment, as it was an early indicator of the unexpectedly high coronal temperature and demonstrated that American astronomers could generate meaningful physical data from solar observations.

Young’s subsequent career would make him one of the foremost American spectroscopists. He published extensively, taught generations of students, and became a bridge between the traditions of positional astronomy and the emerging practice of physical observation.

Other stations met with mixed results. At Mount Pleasant, Wesleyan professor John Van Vleck (grandfather of twentieth-century physicist John Van Vleck) attempted spectroscopic observations using borrowed equipment, but struggled with unfamiliar instruments and unstable mounts. Edward C. Pickering, then a young “rising star” at MIT, conducted a series of eclectic experiments—temperature, polarization, visual spectra—from the window of his hotel room, but his results were inconclusive. In Ottumwa, Princeton’s Stephen Alexander used a thermoelectric pile to measure coronal heat, a novel but unsuccessful attempt to capture infrared radiation during totality.

These experiments, while uneven in their success, underscore a broader point: the eclipse had drawn American astronomers out of their institutional comfort zones and into the domain of experimental physics. Many of them were trying spectroscopy, photometry, or electronic instrumentation for the first time. Even when the results were thin, the shift in approach was significant.

Aftermath and legacy

Bartlett and Hockey show that the effects of the 1869 eclipse rippled outward. For most of the expedition staff, the eclipse marked only a brief foray into physical astronomy. They returned to their computational duties in Washington, and the NAO would not institutionalize physical research. But for others, especially Young and Pickering, the eclipse provided a launchpad.

Pickering in particular would go on to reshape the Harvard College Observatory into a powerhouse of astrophysical research. Under his leadership, HCO became a center for photographic spectroscopy and stellar classification, laying the groundwork for the Henry Draper Catalog and the work of the Harvard Computers.

Meanwhile, Young’s green coronal line became a recurring puzzle in solar physics, leading to improved instrumentation and successive eclipse expeditions, including for the 1870 eclipse in Spain and Algeria. Over time, these observations built a case for understanding the solar atmosphere as a hot, dynamic plasma—an insight that would not be fully appreciated until the twentieth century.

Perhaps the most enduring significance of the 1869 eclipse is how it helped reshape American astronomy’s identity. Before the eclipse, much of US astronomy was defined by its service to state and military needs, such as navigation and timekeeping. Afterward, it began to imagine itself as a science of physical inquiry, one that asked not only where the stars were, but what they were made of.

The shift was neither immediate nor universal. As Bartlett and Hockey note, a debate between traditional and physical astronomy would persist for decades. But the 1869 eclipse made it impossible to pretend that laboratory methods belonged solely to the chemist or physicist. The sky itself had become a laboratory. In this sense, the eclipse was not just an astronomical event, it was a moment of disciplinary transformation. It revealed how quickly a field could evolve when prompted by the right combination of opportunity, funding, and vision.

Rebecca Charbonneau
American Institute of Physics
rcharbonneau@aip.org

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