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dread as the forerunner of the victory of William of Normandy. It was ihen equal to the full moon in size. In 1456, its tail reached from the horizon to the zenith. It was supposed to indicate the success of Mahomet II., who had already taken Constantinople, and threatened the whole Christian world. Pope Calixtus III., therefore, ordered extra Ave Marias to be repeated by everybody, and also the church bells to be rung daily at noon (whence originated the custom now so universal). A prayer was added as follows: "Lord, save us from the devil, the Turk, and the comet." In 1223, it was considered the precursor of the death of Philip Augustus. The first recorded appearance of Halley's comet was B. c. 130, when it was supposed to herald the birth of Mithridates.

The comet of 1843 was Bo intensely brilliant that it was visible in full daylight. It was so near the sun as " almost to graze his surface."

Encke's comet has a period of only 3J years. A most interesting discovery has been made from observations upon its motion. The comet returns each time to its perihelion about 2£ hours earlier than the most perfect calculations indicate. Hence, Prof. Encke has been led to conjecture that space is filled with a thin, ethereal medium capable of diminishing the centrifugal force, and thus contracting the orbit of a comet.

Donati's comet, which appeared in 1858, was the subject of universal wonder. When first discovered, in June, it was 240,000,000 miles from the earth. In August, traces of a tail were noticed, which expanded in October to about 50,000,000 miles in length. This

Fig. 66.

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comet, though small, has never been exceeded in the brilliancy of the nucleus and the graceful curvature of the tail. It will return in about 2,000 years.

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Description.—If we watch the western horizon in March or April, just after sunset, we shall sometimes see the short twilight of that season illuminated by a faint, nebulous light, of a conical shape, flashing upward, often as high as the Pleiades. In September and October, at early dawn, the same appearance can be detected near the eastern horizon. The light can be seen in this latitude only on the most favorable evenings, when the sky is clear and the moon absent. Even then, it will be frequently confounded with the Milky Way or auroral lights. At the base it is of a reddish hue, where it is so bright as very often to efface the smaller stars. In tropical regions the zodiacal light is perpetual, and shines with a brilliancy sufficient, says Humboldt, to cast a sensible glow on the opposite part of the heavens.

Origin.—The commonly received opinion is, that it is caused by a faint cloud-like ring, perhaps a meteoric zone, that surrounds the sun, and only becomes visible to us when the sun himself is hidden below the horizon. Others maintain that, since it has been seen in tropical regions in the east and west simultaneously, it can be explained only on the theory of a "nebulous ring that surrounds the earth within the orbit of the moon."

"He telleth the number of the stars; He calleth them all by their names."

Psalm cxlvii. 4

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