Page:Popular Science Monthly Volume 42.djvu/371

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mass of the comet swept over the surface of Jupiter itself, and that we had here a true collision between the two bodies. The comet struck the planet a glancing blow, as it were. As is usual in all collisions, the weaker body suffered: the comet was broken into three parts, while Jupiter was unharmed.

When the comet had passed far enough away from Jupiter, so that the sun had regained its supremacy, the motion was again referred to the sun as fixed point, and the tedious process of tracing the comet's history continued, step by step. Tracing thus backward the path of this minute body, we find that it leads to the spot where Lexell's comet disappeared in 1779. Either two comets can occupy the same space at the same time, or the comets of 1770 and 1889 are one and the same.

We have thus seen something of the laborious process by which, starting with a few observed positions of a body in 1889, we can trace the path it has traveled for one hundred and seven years; how we can show its identity with a comet seen in 1770. Of the path itself but little has yet been said. It is interesting. And as it is always easier to trace a succession of events in the order in which they occur than it is to reverse the order of time, we will start with the first recorded public appearance of the comet, in 1770, and give a brief sketch of its erratic course through the heavens since that moment—though, remember, this path was discovered by tracing the body backward along its course.

Look again at the diagram. In the summer of 1770 the comet was seen moving along the small dotted curve in the region very close to the smallest circle in the diagram, which represents the orbit of the earth. It disappeared from view and passed outward along this dotted curve, making one complete revolution, returning in 1775 to the point where it was first seen. During these few years the earth had also been traveling its yearly path around the sun, and it so happened that in this latter year (1775) the earth had moved into a different position in its orbit, so that the sun was directly between it and the comet. The comet was therefore not then seen. Onward went the comet along this dotted path, until, in 1779, it had reached the outermost point, when it encountered Jupiter. The effects of this appulse were very marked as regards the comet: it was pushed completely out of its path and set moving in an immense ellipse, the one that extends far out to the left in the diagram. From five and a half years its period had been changed to about thirty-four years. In this large path this captive body moved without any extraordinary incident for sixty-seven years, or until 1846. During this time it had traveled twice around the curve, and it was fairly started on its third trip, when Saturn took a hand in the game and altered its path considerably, extending the ellipse to one of forty-seven years period. On it