Page:Popular Science Monthly Volume 60.djvu/250

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POPULAR SCIENCE MONTHLY.

from our ordinary conception of a gas. That which we see, which gives the sun its apparent size, which sends us our light, is known as the photosphere. This is probably a brilliant shell of metallic clouds floating in an atmosphere of vapors of the same materials. There are certain details in this photosphere with which we are familiar, such as bright patches, known by different names, and sun-spots. For convenience we may regard this photosphere and all that it contains as the Sun, and all that lies outside this shell as the solar atmosphere. With the sun itself we have little to do in this article, since it can be better observed on any clear day than at time of eclipse. It is, however, only at time of total eclipse that we clearly see all those strange and complex features which make up what we have called the solar atmosphere. In our study of it, however, we must not be governed too much by any analogy with our own atmosphere. Lying next to the body of the sun is a layer of crimson flame, known as the chromosphere, which has a thickness of perhaps 5,000 or 6,000 miles. This may seem like a great depth for such a sea of fire, but compared with the enormous size of the sun it is very small indeed, and forms but a thin rose-colored rim about the edge of the sun. At the bottom of this is probably the so-called reversing layer. The solar spectrum is crossed by dark lines due to the elements which there exist. By these dark absorption lines, which are seen in the ordinary solar spectrum, the presence is known of many familiar elements. The higher regions of the chromosphere are less complex and consist in large part of hydrogen. From these regions, by forces which there operate, great masses of brilliant colored gas are thrown upward to enormous distances, in general 10,000, or 20,000 miles, but often much higher, even to 200,000 or 300,000 miles. Resting also on the photosphere is the corona, which extends its pearly light outward from the sun to immense distances which must be reckoned in millions of miles.

The different parts of the solar atmosphere are brightly luminous, and stand forth in splendid beauty at the instant of totality. The only reason why we do not see them on any clear day is that they are lost in the blinding light of the central sun. The sun's face must be shut out. This service is rendered by the moon at an eclipse. At other times the chief trouble is not that the sun shines directly into our eyes, since a piece of cardboard could be so placed as to cut off the rays. The real difficulty arises from the presence of our atmosphere, which becomes so bright from the diffused light of the sun, that the solar appendages are lost to view. This will be apparent from the daily phenomenon of the appearance by night, and the disappearance by day, of the stars. They are shining just as brightly by day as by night, and could be seen perfectly well if the atmosphere were removed for a moment.