Scientific Investigations

For the professional astronomers, the main subject of inquiry was the solar corona. The nature of that structure, seen only during an eclipse, was as yet unclear. What is it made of? Does it contain yet more unknown chemical elements, like helium? These are the sorts of questions we addressed in Chapter 5. Back in 1878, scientists had real quandaries with understanding the corona. These are nicely encapsulated by the following passage that appeared in the Boston Globe: "The corona is not a solar atmosphere in the sense in which the word is usually understood, since the great comet of 1843 passed through 300,000 miles of it at the enormous velocity of350 miles per second without suffering visible damage, or being in the least retarded; yet shooting stars passing through the upper portions of the Earth's atmosphere are completely vaporized, although their speed never exceeds fifty miles per second. What then is the corona?"

The problem with the corona was based upon its dimensions. Although its measured size alters from eclipse to eclipse due to variations in solar activity, typically it stretches up above the photosphere by a distance of the same order as the radius of the Sun. That means it is huge. The atmospheres of the Earth and Mars were known to be of very limited extent, only tiny fractions of the radius of either planet, and so astronomers were at a loss to explain the vast corona they saw. Even given that they knew that the solar photosphere was very hot, many thousands of degrees, still they could not explain the corona's extent. The calculations indicated that if it were as hot as the glowing solar surface, then the corona might extend out into space for a few hundred miles—but not half a million miles.

It was known that the corona must be very tenuous, else it would be apparent at other times than during eclipses. Limits on its thickness, in terms of the amount of light it absorbs, were set in 1878 when an observing team from Chicago noted that they could see one of the stars in the constellation Cancer right through the corona. Thus although it appears bright during an eclipse, the corona must contain very little matter, else it would have absorbed the star's light.

The eventual solution came when it was realized that the temperature of the corona is measured not in thousands of degrees, but in millions. In 1878 this was entirely unsuspected. Various specific experiments were planned to probe its nature during the eclipse. By using spectroscopes it was hoped to identify new constituents. Using polarimeters, which enable the polarization of the light received to be determined, astronomers hoped to be able to identify whether there was dust within the corona. Nowadays it is known that there is an enhancement in the density of interplanetary dust near the Sun which may be investigated by using the sunlight it scatters, and this is termed the F-corona, but a century and more ago there was scarce knowledge of such things.

Regarding the temperature of the corona, an attempt was made to measure its heat directly. These were still early days in infrared astronomy and very little was understood. But the right man to have on that task was Thomas Alva Edison, the great inventor.

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