ful telescope. The sun has a diameter of 860,000 miles and, as its distance from the earth is only 93,000,000 miles, an extremely small fraction of the distance of the other stars, it is possible to observe and to study in detail its extraordinary phenomena, which are incomparably more violent than anything observed on the earth. When we speak of the sun we speak collectively of a great number of phenomena, some of which extend for millions of miles from the sun's visible disk. Chief of these is the corona, a vast filmy atmosphere so rare that it offers little or no resistance to the passage of a comet, as it sweeps around the sun under the action of gravitation and returns into the space from which it came. The polar streamers of the corona (Fig. 9) suggest the
Fig. 9. The Solar Corona.
Photographed by Yerkes Observatory Eclipse Expedition, May 28, 1900 (Barnard and Ritchey).
action of magnetic forces and offer material for long continued study of this, the most mysterious of all the solar appendages. At the base of the corona, rising out of a sea of flame which completely encircles the sun, are the prominences, some of which occasionally attain a height of nearly 400,000 miles. Like the corona, the prominences are hidden by the brilliant illumination of our own atmosphere, and are visible to the naked eye only when the direct light of the sun's disk is cut off by