171. METEOR SHOWERS--THE RADIANT.--There is evident among meteors a distinct tendency for individuals, to the number of hundreds or even hundreds of millions, to travel together in flocks or swarms, all going the same way in orbits almost exactly alike. This gregarious tendency is made manifest not only by the fact that from time to time there are unusually abundant meteoric displays, but also by a striking peculiarity of their behavior at such times. The meteors all seem to come from a particular part of the heavens, as if here were a hole in the sky through which they were introduced, and from which they flow away in every direction, even those which do not visibly start from this place having paths among the stars which, if prolonged backward, would pass through it. The cause of this appearance may be understood from Fig. 112, which represents a group of meteors moving together along parallel paths toward an observer at _D_. Traveling unseen above the earth until they encounter the upper strata of its atmosphere, they here become incandescent and speed on in parallel paths, _1_, _2_, _3_, _4_, _5_, _6_, which, as seen by the observer, are projected back against the sky into luminous streaks that, as is shown by the arrowheads, _b_, _c_, _d_, all seem to radiate from the point _a_--i. e., from the point in the sky whose direction from the observer is parallel to the paths of the meteors.
[Illustration: FIG. 112.--Explanation of the radiant of a meteoric shower.--DENNING.]
Such a display is called a meteor shower, and the point _a_ is called its radiant. Note how those meteors which appear near the radiant all have short paths, while those remote from it in the sky have longer ones. Query: As the night wears on and the stars shift toward the west, will the radiant share in their motion or will it be left behind? Would the luminous part of the path of any of these meteors pass across the radiant from one side to the other? Is such a crossing of the radiant possible under any circumstances? Fig. 113 shows how the meteor paths are grouped around the radiant of a strongly marked shower. Select from it the meteors which do not belong to this shower.
[Illustration: FIG. 113.--The radiant of a meteoric shower, showing also the paths of three meteors which do not belong to this shower.--DENNING.]