Page:Popular Science Monthly Volume 24.djvu/496

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480 THE POPULAR SCIENCE MONTHLY.

Our ring, incomparably smaller, covers only a narrow zone of the polar regions, the center of which is at a considerable distance from the pole. The inhabitants of Saturn's equator—if there are any—look out upon a ribbon very wide in the vertical but very narrow in the horizontal direction. On the other hand, an observer in the high latitudes of Asia or America stands in the presence of a corona of little thickness, but relatively extensive; that is, the development of our ring is nearly parallel to the part of the terrestrial surface dominated by it, and which it would overshadow if it were opaque.

To this theory the objection may be offered, that no one before M. Nordenskjöld has remarked the meteor in question, while many should have done so if it is permanent. An observer standing near the auroral pole should perceive a luminous circle completely enveloping the horizon. M. Nordenskjold replies to this by saying that the luminous arc is only a residuum of more brilliant and more complex phenomena; we can hardly hope to see it except in years when auroras are weak, or years of minima, of which the year 1878-'79 was one. Most commonly the accessory masks the principal, much in the same way that we can not see the foundations of a house while the building is standing. The light of the ring is so weak that not only the day and the twilight, but simple moonlight makes it invisible. If the sky is charged with frost, it will all disappear, and even the presence of too much vapor in the air extinguishes it. The observer must, then, be favored with dry and cold weather. If the temperature is above the freezing-point, it is useless to look for the corona. The coasts of Norway, moist with the breezes from the Gulf Stream, are badly situated to give views of it. Nearly all other regions where it could be perceived are dismal solitudes. In the second place, a spectator situated near the auroral poles would see nothing, for the horizon would hide the meteor from him in the same way that a Saturnian, who never left the high polar regions of his planet, would not be aware of the existence of his ring. Our observer, leaving the auroral pole, and going toward the magnetic south, would finally distinguish in that direction an arc gradually rising above the horizon. An entire circle of considerable width is dominated—that is the word—by the corona, which is then near the zenith; but, although the meteor may be nearer the ground at that point than anywhere else, it is not visible there, for it is too thin to be seen, looking at it vertically. Outside of this latter zone, another zone, concentric with it, enjoys the sight of the arc, now situated obliquely in the direction of the magnetic north. Further on, the arc, grazing the horizon, ceases to be visible; some time be- fore reaching this point, in fact, it is hidden by the mists that gather in the horizon, as well as by the density of the atmosphere which the visual rays have to traverse. M. Nordenskjöld would not have been able to see it if it had been only half as luminous.

The meteor is relatively stationary, but is not rigorously motionless.