“This comet has helped to throw light on one of the most perplexing of all the puzzles which those most perplexing of all the heavenly bodies, comets, have presented to astronomers. In the year 1668, a comet was seen in the southern skies which attracted very little notice at the time, and would probably have been little thought of since had not attention been directed to it by the appearance and behavior of certain comets seen during the last half-century. Visible for about three weeks, and discovered after it had already passed the point of its nearest approach to the sun, the comet of 1668 was not observed so satisfactorily that its orbit could be precisely determined. In fact, two entirely different orbits would satisfy the observations fairly, though only one could be regarded as satisfying them well. “This orbit, however, was so remarkable that astronomers were led to prefer the other, less satisfactory though it was, in explaining the observed motions of the comet. For the orbit which best explained the comet’s movements carried the comet so close to the sun as actually to graze his visible surface. Moreover, there was this remarkable and, indeed, absolutely unique peculiarity about the orbit thus assigned: the comet (whose period of revolution was to be measured by hundreds of years) actually passed through the whole of that part of its course during which it was north of our earth’s orbit plane in less than two hours and a half! though this part of its course is a half-circuit around the sun, so far as direction (not distance of travel) is concerned. That comet, when at its nearest to the sun, was traveling at the rate of about 330 miles per second. It passed through regions near the sun’s surface commonly supposed to be occupied by atmospheric matter. “Now, had the comet been so far checked in its swift rush through those regions as to lose one-thousandth part of its velocity, it would have returned in less than a year. But the way in which the comet retreated showed that nothing of this sort was to be expected. I am not aware, indeed, that any anticipations were ever suggested in regard to the return of the comet of 1668 to our neighborhood. It was not till the time of Halley’s comet, 1682, that modern astronomy began to consider the question of the possibly periodic character of cometic motions with attention. (For my own part, I reject as altogether improbable the statement of Seneca that the ancient Chaldean astronomers could calculate the return of comets. The comet of 1680, called Newton’s, was the very first whose orbital motions were dealt with on the principles of Newtonian astronomy, and Halley’s was the first whose periodic character was recognized.)