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The Trans-american Eclipse Of 1869

Presentation #306.03 in the session “History of Astronomy”.

Published onJun 18, 2021
The Trans-american Eclipse Of 1869

In honor of an AAS meeting in conjunction with the SPD, I present a (COVID-delayed) celebration of 1869’s solar-eclipse sesquicentennial. I refer to the earliest total eclipse of the Sun in which totality spanned the USA (Washington Territory to North Carolina). Moreover, it was the first time in our country’s history in which the total-eclipse milieu had all the hallmarks of a contemporary eclipse: professional expeditions to the path, scientific results, public enthusiasm, and hucksters ready to exploit the episode.

Yet 7 August 1869 is a vivid example of the manner in which events, at the time of great importance, fade in memory with the decades. They meld into a faint and obscure background, replaced by interest in more recent affairs . . .

The last total solar eclipse, to pass over (what would become) an extensive portion of the United States, had occurred nearly thirty years earlier: 5 September 1840. At the time, much of these lands belonged to Mexico. Even if the term ‘eclipse expedition’ had had any well-defined meaning then, an American effort to place astronomers on the track would have been nearly impossible.

But 1869 should be famous in science for a reason beyond time and geography: The familiar apparatuses of astronomy, undertaken in support of the navigator, were fairly uncomplicated and had not substantially changed for hundreds of years. They were devices with which to gauge small angles and, later, clocks with which to determine precise time.

However, heavy baggage now was required, because solar-eclipse expeditions no longer were focused on positional astronomy. The 1869 total eclipse of the Sun was the initial overtly astrophysical eclipse in the maturing nation. Using newly developed technology, such as the spectroscope and camera (in addition to such things as photometers, polarimeters, etc.), scientists had become interested in the very nature of the phenomena they witnessed. The necessary instruments of physical astronomy were bulky, delicate, and cost substantial amounts. Transport by steam train was the best way to get astronomers and gear safely to the site.

This now was possible due to a coincidence. The optimum total-eclipse observing station was the intersection of the umbral path with the westernmost depot of the current railway network. In 1869, the Golden Spike had just been hammered into a rail. Suddenly, one easily could truck freight loads of delicate equipment as far as the Iowa frontier, to encounter the long-duration totality and anticipated clear skies required by the undertakings listed above. The best eclipse ‘observatory’ became any unobstructed lot close to a railroad platform.

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