Page:Popular Science Monthly Volume 42.djvu/366

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ON the night of June 14, 1770, the great French astronomer Messier first saw the captive comet. It then appeared as a small patch of haze against the cloudless sky, but it rapidly grew larger and more brilliant, until, on July 2d, when it passed nearer to the earth than any other known comet, it was as bright as the North Star, and its diameter was twice that of the full moon. From that moment its brilliancy faded, it grew fainter and fainter, and was seen for the last time on October 2d.

While this comet of 1770 is one of the most famous in the annals of astronomy, it owes its celebrity not to the spectacular effects it produced, for it was not one of those magnificent objects that stretch across the heavens, exciting the wonder and admiration of the intelligent, the fear and dread of the ignorant. Its fame is due to its mathematical history, to the path it was then traveling, and to the path it has since traveled. Only twenty years had elapsed since Halley had made his great discovery of the existence of periodic comets, and this comet of 1770 was shown by Lexell to belong to this interesting class of bodies, to be then revolving around the sun in an ellipse of five and a half years. To the conclusions of Lexell it was at once objected by other mathematicians that if this comet revolved about the sun in an ellipse, like the planets, it should have been seen six years before, and again, six years before that; at least, some record of its former appearances ought certainly to be found. As there were no such records, as it could be shown that there was no comet that had appeared regularly every five and a half or six years, Lexell's opinions were for the moment discredited. However, he soon conclusively proved that he was right, that the comet was moving at the moment in an ellipse such as he had described, but that it had not always traveled in that same path. He showed that in 1767, or only three years previously, the comet had passed very close to the giant planet Jupiter, and that then its path had been greatly altered, so completely changed, indeed, that never before had it passed near enough to the earth to be seen. He also predicted a second close approach of these two bodies in 1779, and said that this circumstance might prevent the reappearance of the comet after that date. This prediction of Lexell's was fulfilled, for the comet was never again seen, unless it prove that the comet discovered by Brooks on July G, 1889, is the lost body.

On that summer evening, at Geneva, N. Y., Brooks discovered a faint telescopic comet, since known as comet d and V, 18S9. As this body never became visible to the naked eye, it received but