Page:Popular Science Monthly Volume 11.djvu/367

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353
THE ZODIACAL LIGHT.

Cassini discovered very soon that as time advanced through March and April, the upper or northern edge of this light inclined more and more from the ecliptic, and stretched farther to the northward; and knowing that the sun's equator, as shown by his spots, was also then stretching off from the ecliptic in a similar way, he came to the conclusion that the substance giving this light was closely connected with the sun's equator, and was consequently changing its position with regard to that equator.

He argued, further, that as the sun has an atmosphere, and is therefore capable of emitting dense vapors, and is continually sending out matter of exceeding fineness which we call light, consequently this luminary might also, by its motion on its axis, send out a substance intermediate in character between the two; which substance, either self-luminous or by reflection, might give us the zodiacal light. To support this theory, he gives to this body a lenticular shape, about twice the thickness of the sun as seen in March, but only of the sun's thickness when seen in June. Whether he meant to have this lenticular-shaped medium to be attached to the sun, and revolving with it at the same time, is not apparent. He devoted a part of his time for about eleven years, in a very desultory manner, to observing this light.

Cassini's labors led other observers to direct their attention to the zodiacal light. Fabio de Duillier, who was his colleague for a while at Paris, is worthy of particular notice, as having conceived the idea that it consisted of particles of matter distinct from the sun, and arranged in shape like a solid zodiac; which body of uneven surfaces, and rotating round the sun, he supposed, gave us the zodiacal light.

In 1731 Mairan gave considerable attention to this subject in a work on "The Aurora Borealis." He advanced the theory that this light is reflected from the sun's atmosphere, stretched out into a flattened spheroid or lenticular-shaped body, revolving with the sun—an idea which Laplace has forever set at rest by demonstrating that the sun's atmosphere can extend no farther than to the orbit of a planet whose periodical revolution is performed in the same time as the sun's rotary motion about its axis, or in twenty-five days and a half; that is, only so far as nine-twentieths of Mercury's distance from the sun.

Since the time of Mairan, until 1853, but little attention appears to have been given to this subject. In 1833, however, Biot, in order to account for the meteor-shower of that year, attempted to show that the shower was owing to the earth's passing at that time near the node of the zodiacal light. But calculations were made by J. C. Housseau in order to see whether the nodes of the sun and the zodiacal light do actually correspond. After careful observations and calculations, in which he was assisted by nine diligent observers, Housseau thinks he has shown that these nodes are different, and that