Page:Popular Science Monthly Volume 16.djvu/268

254 The predictions made in November, 1872, were not so precise as they would probably have been if the comet had been seen in 1866 and in 1872, as had been expected. Indeed, astronomers had very little experience as to the meteors of Biela's comet. They were in doubt what showers among those recorded by various observers of meteors as occurring during the last week of November and the first week of December could be associated with this particular meteor system. For until the astronomical significance of meteoric displays had been fully recognized, the observers of shooting stars, even when these were seen in showers, had been more careful to record the brightness and the number of the meteors than their course among the stars. So that the criterion which at present distinguishes one meteor system from another, even though both meteor systems may show falling stars on one and the same night or at one and the same time, is not applicable to most of the records of star-showers. That criterion, it need hardly be said, is the position of what is called the radiant point of the star-shower, the point from which all the meteor-tracks on the sky seem to tend. The reader must not fall into the mistake of supposing that every meteor-track absolutely extends from the so-called radiant. On the contrary, it may truly be said that not one such track does or can extend from that point. But each tends from the point in the sense that, if the course pursued by the meteor be supposed to be extended backward in a straight line (or, more correctly speaking, in a great circle of the heavenly sphere), the line would pass through the radiant point. The expression is used in the same general sense, and has, in fact, the same significance as the statement usually made about parallel lines and their vanishing point in perspective. Lines which are really parallel are so drawn in perspective that they all tend from one and the same point, but they do not extend from it. An artist might indeed draw them all in pencil from that point, but he would afterward rub out parts of the pencil-lines, leaving the rest all tending from the vanishing-point, but none of them extending actually from it.

Now, what is the radiant point of a meteor system? It is in reality that infinitely remote point from which all the meteors seem to be traveling—the point toward which all the parallel lines on which they are actually traveling seem to converge. No meteor, then, approaching the earth on the course thus indicated could possibly seem to move actually from the radiant point. If moving directly toward the observer, it would be visible at the radiant point, all the time, not seeming to move from it; if not moving directly toward the observer, but on a course parallel to that from the radiant point to the observer, it would be seen, from the beginning to the end of its flight, at points removed from the radiant, but all on a line tending from it. Thus the