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least six diameters of the moon, though the eye has traced it farther. This corona is still one of the wonders of astronomy, and leads to many questions. What is its consistency, if it extends many million miles from the sun's surface? How is it that it opposed no resistance to the motion of comets which have almost grazed the sun's surface? Is this the origin of the zodiacal light? The character of the corona in photographic records has been shown to depend upon the phase of the sun-spot period. During the sun-spot maximum the corona seems most developed over the spot-zones — i.e., neither at the equator nor the poles. The four great sheaves of light give it a square appearance, and are made up of rays or plumes, delicate like the petals of a flower. During a minimum the nebulous ring seems to be made of tufts of fine hairs with aigrettes or radiations from both poles, and streamers from the equator.
On September 19th, 1868, eclipse spectroscopy began with the Indian eclipse, in which all observers found that the red prominences showed a bright line spectrum, indicating the presence of hydrogen and other gases. So bright was it that Jansen exclaimed: "Je verrai ces lignes-la en dehors des eclipses." And the next day he observed the lines at the edge of the uneclipsed sun. Huggins had suggested this observation in February, 1868, his idea being to use prisms of such great dispersive power that the continuous spectrum reflected by our atmosphere should be greatly weakened, while a bright line would suffer no diminution by the high dispersion. On October 20th Lockyer,1 having news of the eclipse, but not of Jansen's observations the day after, was able to see these lines. This was a splendid performance, for it enabled the prominences to be observed, not only during eclipses, but every day. Moreover, the next year Huggins was able, by using a wide slit, to see the whole of a prominence and note its shape. Prominences are classified, according to their form, into "flame" and "cloud" prominences, the spectrum of the latter showing calcium, hydrogen, and helium; that of the former including a number of metals.
The D line of sodium is a double line, and in the same eclipse (1868) an orange line was noticed which was afterwards found to lie close to the two components of the D line. It did not correspond with any known terrestrial element, and the unknown element was called "helium." It was not until 1895 that Sir William Ramsay found this element as a gas in the mineral cleavite.
The spectrum of the corona is partly continuous, indicating light reflected from the sun's body. But it also shows a green line correspond
1 Acad, des Sc., Paris; C. R., lxvii., 1868, p. 121.
ing with no known terrestrial element, and the name "coronium" has been given to the substance causing it.
A Vast number of facts have been added to our knowledge about the sun by photography and the spectroscope. Speculations and hypotheses in plenty have been offered, but it may be long before we have a complete theory evolved to explain all the phenomena of the storm-swept metallic atmosphere of the sun.
The proceedings of scientific societies teem with such facts and "working hypotheses," and the best of them have been collected by Miss Clerke in her History of Astronomy during the Nineteenth Century. As to established facts, we learn from the spectroscopic researches (1) that the continuous spectrum is derived from the photosphere or solar gaseous material compressed almost to liquid consistency; (2) that the reversing layer surrounds it and gives rise to black lines in the spectrum; that the chromosphere surrounds this, is composed mainly of hydrogen, and is the cause of the red prominences in eclipses; and that the gaseous corona surrounds all of these, and extends to vast distances outside the sun's visible surface.
13. The Moon And Planets.
The Moon. — Telescopic discoveries about the moon commence with Galileo's discovery that her surface has mountains and valleys, like the earth. He also found that, while she always turns the same face to us, there is periodically a slight twist to let us see a little round the eastern or western edge. This was called libration, and the explanation was clear when it was understood that in showing always the same face to us she makes one revolution a month on her axis uniformly, and that her revolution round the earth is not uniform.
Galileo said that the mountains on the moon showed greater differences of level than those on the earth. Shroter supported this opinion. W. Herschel opposed it. But Beer and Madler measured the heights of lunar mountains by their shadows, and found four of them over 20,000 feet above the surrounding plains.
Langrenus1 was the first to do serious work on selenography, and named the lunar features after eminent men. Riccioli also made lunar charts. In 1692 Cassini made a chart of the full moon. Since then we have the charts of Schroter, Beer and Madler (1837), and of Schmidt, of Athens (1878); and, above all, the photographic atlas by Loewy and Puiseux.
The details of the moon's surface require for their discussion a whole book, like that of Neison
1 Langrenus (van Langren), F. Selenographia sive lumina ausiriae philippica; Bruxelles, 1645.