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Solar Astronomy in 1868
Darkness still amazes astronomers today as much as it did over 140 years ago in 1868, since it is rare to experience this darkness that comes with a total solar eclipse. Currently, scientists use solar eclipses to try to achieve an accurate measure of the diameter of the sun, study the sun's atmosphere, and search for astronomical bodies near or behind the sun. And these were the exact same goals during a total eclipse on August 18, 1868.
This eclipse, which crossed over Ethiopia, India, the Malay Peninsula (then known as the Malacca Peninsula), Indonesia, and Papua New Guinea, lasted from two to eight in the morning. At totality over Cambodia, the darkness lasted six minutes and forty-six seconds--a good amount of time for observation. This was a relatively long duration compared to other eclipses, due to the sun's being nearly at its farthest point from Earth (apogee) and the moon being nearly at its closest point to Earth (perigee).
Now I shall take a moment to deviate. Recently, I was researching at the local library when I found a magazine from 1868 created and edited by Charles Dickens, called All the Year Round. Inside was an article, entitled "The Coming Eclipse," written in anticipation of the August 18 total eclipse. Little did the author of the article know when he wrote, "We shall probably have a more approximate answer to the much-vexed question, What is the Sun?" For what came of this eclipse became highly significant in the study of the sun--and the Earth. French astronomer Pierre Janssen discovered helium.
Janssen was sent as part of an expedition from France, costing two thousand pounds, in Victorian era English currency. The team observed the eclipse from India, in order to avoid possible rain in eastern Asia, as it was monsoon season. During the eclipse, Janssen studied the spectrum of a solar prominence, a bright cloud of ionized gases extending from the sun's chromosphere. In the spectrum of the prominence, the astronomer noticed a yellow line with a new wavelength, one different from the well-known wavelength of sodium.
Janssen suspected the line to represent a new element, and he reported it to the English astronomer Joseph Norman Lockyer, who had missed the eclipse. When Lockyer failed to determine if the spectral line came from a known element, he dubbed it a new element, helium, named for the Greek word for the sun. Three decades later, in 1895, helium was finally discovered on Earth (since helium tends to fly away before the scientists can get a handle on it).
Bibliography
Dickens, Charles, ed. "The Coming Eclipse." All the Year Round 1 Aug., 1868: 185-7.
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