Page:Popular Science Monthly Volume 42.djvu/367

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passing notice from the daily newspapers; even astronomers, at first, thought very little of it, as the discovery of a new comet is now a matter of almost monthly occurrence. It was not long, however, before this body began to attract the attention of the scientific world, and it was soon recognized as a permanent member of the solar system; and now, through the researches of Dr. Chandler and others, it has become the most famous comet of this century. It has been identified with the lost comet of Lexell, which disappeared one hundred and twenty years ago.

Upon what grounds do we base this conclusion? A comet was seen for but a few months during the summer of 1770, another one is observed during the summer and fall of 1889, and it is asserted that these two bodies are identical. There are no physical means by which they can be identified, for comets have no permanent characteristics which, when once seen, can always be recognized. Indeed, to all appearances, these two bodies were utterly unlike: the comet of 1770 was large and bright, with a well-marked tail; while the comet of 1889 was hardly visible even with powerful telescopes, and then appeared but as a small patch of haze against the dark sky. If, then, we rely on similarity of appearance to establish the identity of these two comets, we should fail to do so, and would be forced to conclude that they are not the same. But by a study of the movements of the two, especially of the latter, it can be shown that they must have occupied, at one time, the same position in space—their identity is then self-evident.

At present the comet is moving in a small ellipse of about seven years' period. This path is shown in the diagram. The smallest circle represents the annual orbit of the earth around the sun. Just outside of this circle is a heavily drawn ellipse with one of its foci at the sun. This is the present orbit of comet V, and on it are marked three positions of this interesting body. The first, July 6, 1889, marks its position on the night of discovery; the second, September 30th, at its perihelion passage, or nearest approach to the sun; the third, that of December, 1890, the position it occupied when last seen. For months before this last date, however, the comet could only be seen by means of the great thirty-six-inch Lick telescope. Between the two extreme positions above mentioned, there are scattered along the curve some two hundred and fifty other observations; and on this small part of the comet's path rest all the conclusions as to its movements for over a hundred years.

The first step in the problem was to deduce from these observed positions the orbit of the comet, or the ellipse shown in the diagram. This curve should be clearly understood—it is not the actual path of the comet through the heavens, but that path