Eclipse Vicissitudes:Thomas Edison and the Chickens
Last August 11 my wife and I found ourselves in the countryside of southern Hungary preparing to watch a total eclipse of the Sun. Luck was with us; an overcast, rainy morning gave way to clear skies an hour or two before totality began near midday. Although it wasn't our first total solar eclipse, we were nevertheless caught up in the fascination the event always brings: the increasingly eerie quality of the dying light as the moon relentlessly gnaws its way across the face of the sun, the expectant hush of the staring people around us, the sudden cries of awe as, in this case, the diamond ring phenomenon suddenly flashes into view. And with the closing darkness the local birds, true to form, set off to roost in the nearby trees, twittering in bewilderment at the early coming of night. I was immediately reminded of the story of Thomas Edison and the chickens.
It's a story that centers on the total eclipse of July 29, 1878, an eclipse notable on more than one historical front. I have already written about it (Marginalia, September–October 1994) in connection with the search for a putative planet, Vulcan, thought to revolve between the Sun and Mercury. Vulcan's presence, it had been hoped, would explain a discrepancy of about 40 arcseconds per century between theory and observation in the orbital motion of Mercury.
It was an eclipse well above average in interest to astronomers. The path of total eclipse, starting in Alaska, was to sweep down the spine of the Rocky Mountains and out across the Gulf of Mexico before ending in Cuba. This meant that it might be observed from an area with generally clear weather containing many high-altitude mountainous sites, mostly above the haze and water vapor that plague lower sites. This fact took on added importance in that the later 19th century was a time in which astrophysics, the study of stars themselves, as distinct from their positions and motions, was beginning an explosive growth. So it was that Samuel Langley, director of the Allegheny Observatory, intended to use these auspicious circumstances to further investigate the strange outer atmosphere of the sun called the corona. A recent theory at the time proposed that the corona, then visible only at a total solar eclipse, was the moon's atmosphere rendered visible by backlighting from the sun. Langley wanted to find out more about the corona and, in particular, its temperature. But how to measure the temperature of something 150 million kilometers away? The obvious thought was that if the corona is hot, as one would expect if it were solar, it must produce infrared radiation, albeit difficult to measure at that distance. This was a real problem for Langley, since existing thermopiles were generally too insensitive to yield a useful result. And it was here that Thomas Edison entered the story.
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