The line of totality stretches from the North Atlantic across the south of Spain, passing over Cadiz, then over Algeria, thence over Syracuse, in Sicily, into Turkey, Greece, etc. The duration of the eclipse will not vary much from two minutes at any place, too short to make the eclipse a very important one, and still less so from the low altitude of the sun. Congress has appropriated nearly $30,000 to equip observers, and we believe, after a good deal of pressure, the British Government has provided a ship and $10,000 for a similar purpose. The astronomical data derivable from a solar eclipse are of two kinds : one respecting the moon’s motions, the other the physical constitution of the sun. The first is not important just now, but we may observe that the moon, coming against or in front of the sun, allows an observation of our satellite’s place in the heavens to be made at a very central point in her orbit. This is of importance to mathematical astronomy, but can only be properly made at a fixed observatory, which need not be on the line of totality.
The expeditions above referred to must devote themselves to the acquirement of physical and cosmical knowledge, from appearances which are only shown when the disk of the moon completely hides the disk of the sun, and permits the view of certain solar surroundings, otherwise too faint to be seen. These surroundings are of two-fold character. First, there is a silvery white glowing radiation of considerable breadth, which encircles the moon, apparently, but doubtless the sun really. This has been seen in eclipses from the earliest times, and is known as the “corona;” second, there is, close to the sun, a narrow rim of bright red excrescences, first seen during an eclipse in 1706, supposed to be an atmosphere round the moon. This phenomenon seems to have been forgotten, until observers of the eclipse of 1842 saw, at the instant of totality, flame-colored masses of light protruding from the lunar disk. From that time this light became the chief feature of a solar eclipse. In 1860, it was settled by aid of photography that this light came from the sun. In 1868, the spectroscope was used, and showed that these red excrescences are part of a shell of glowing gas surrounding the solar globe. Since then, this light has been seen without an eclipse, even in the dazzling rays of the sun, owing to properties of the spectroscope in dilating the heterogeneous light coming from the sun’s general surface, while it allows to pass unaffected the homogeneous light emanating from gaseous bodies, such as these excrescences prove to be, which were seen by turning the spectroscope to the edge of the sun. The sun was virtually put out by spreading its light over such an area as to make the resulting brightness less than that of the prominences — the light of which will not spread itself— and then these were clearly seen. These prominences are but the higher parts of an irregular and tempestuous atmosphere of glowing gas, which cover the whole globe of the sun.
What are familiarly known as “Baily’s beads” no longer attract the attention of eclipse observers. They seem due simply to the sun’s light glancing between the mountains on the edge of the moon, just before totality. The “corona” will be the absorbing phenomenon, doubtless, in the present eclipse. Theories concerning it exist in plenty. Some call it a glare in our own atmosphere, but the whole of it cannot arise from this cause. Some regard it as an atmosphere of the moon, but this idea is quite untenable. It may be an atmosphere about the sun; it may be a dense portion of the zodiacal light; but these are hypotheses. For a fact, we only know that a bright part of the light near the sun seems to be tolerably persistent; and that fainter outlying parts, which shoot out two or three times the sun’s diameter, seem to vary their form during an eclipse, and to present different appearances to different observers. To acquire more facts, we have photography, the spectroscope, and the polariscope. The former of these will probably determine whether the silvery light comes from a luminous gas, or from solid matter in a state of incandescence, or it will show if both qualities of light are present. The polariscope will determine whether the light is the corona’s own self, generated luminosity, or reflected sunlight. To analyze the light of the corona, and attempt to decide its source, must be the main ends of the coming observations. Chiefly to solve this eclipse enigma have we sent some of our highest official philosophers to Europe, and the result of their labors will be awaited with much interest.
Scientific American, 1871, vol. 24, n. 1