< Page:Popular Science Monthly Volume 11.djvu
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THE ZODIACAL LIGHT.

other stellar bodies. Traversing space with inestimable velocity and performing their revolutions with unvaried regularity, they have long been known as part of our solar system. It seems strange that a cosmical body so near the earth as the zodiacal light should have received comparatively so little attention.

As many who read this article have never seen this light, it is necessary that it be described.

It is defined, in the work under review, to be "a brightness that appears in the western sky after sunset, and in the east before sunrise; following nearly or quite the line of the ecliptic in the heavens, and stretching upward to various elevations according to the season of the year." There is a slight objection to this definition, by which inexperienced observers may be led astray. It is spoken of as appearing after sunset, by which some would be led to suppose that it is visible immediately after sunset; whereas it is never to be seen until after the night has fully set in, and the sun's rays are some distance below the horizon. Its varied elevation, indeed its appearance, is also dependent upon the latitude of the observer as well as the season, so much so, that in very high latitudes it is but seldom seen to advantage. It has also been seen at almost every hour of the night, but is usually more distinctly seen in the temperate zones, when observed between dark and nine o'clock, as after that hour its light frequently becomes dim and diffuse.

"It appears to best advantage when the ecliptic makes its highest angle with the spectator's horizon, at which time, in moderate latitudes, it reaches to the zenith or beyond it, having near the horizon a striking brilliancy, and thence fading upward, mostly by imperceptible degrees, till at its vertex it can be made out only by a careful and experienced eye. As the seasons advance, when the ecliptic is declining gradually toward the horizon, the zodiacal light fades away till it is perhaps entirely lost to view, or can be seen only by those who have followed it in its changes, night after night, and are thus able, by familiar acquaintance, to detect and trace its dim markings in the sky."

Humboldt, in "The Cosmos," vol, i., remarks:

"Those who have been for many years in the zone of palms must retain a pleasing impression of the mild radiance with which the zodiacal light, shooting pyramidically upward, illumines a part of the uniform length of tropical nights. I have seen it shine with an intensity of light equal to the milky-way in Sagittarius, and that not only in the rare and dry atmosphere of the summit of the Andes, at an elevation of from 13,000 to 15,000 feet, but even on the boundless grassy plains, the llanos of Venezuela, and on the sea-shore beneath the ever-clear sky of Cumana."

Prof. Olmsted, in his "Astronomy," describes it as follows:

"Its form is that of a luminous pyramid, having its base toward the sun. It reaches to an immense distance from the sun, sometimes even beyond the orbit of the earth. It is brighter in the parts nearest the sun than in those that are more remote, and terminates in an obtuse apex, its light fading away by insen-
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