[Illustration: Fig. 352.] 311. _Meteoric Showers._--On ordinary nights only four or five shooting-stars are seen in an hour, and these move in every direction. Their orbits lie in all possible positions, and are seemingly scattered at random. Such meteors are called _sporadic_ meteors. On occasional nights, shooting-stars are more numerous, and all move in a common direction. Such a display is called a _meteoric shower_. These showers differ greatly in brilliancy; but during any one shower the meteors all appear to radiate from some one point in the heavens. If we mark on a celestial globe the apparent paths of the meteors which fall during a shower, or if we trace them back on the celestial sphere, we shall find that they all meet in the same point, as shown in Fig. 352. This point is called the _radiant point_. It always appears in the same position, wherever the observer is situated, and does not partake of the diurnal motion of the earth. As the stars move towards the west, the radiant point moves with them. The point in question is purely an effect of perspective, being the "vanishing point" of the parallel lines in which the meteors are actually moving. These lines are seen, not in their real direction in space, but as projected on the celestial sphere. If we look upwards, and watch snow falling through a calm atmosphere, the flakes which fall directly towards us do not seem to move at all, while the surrounding flakes seem to diverge from them on all sides. So, in a meteoric shower, a meteor coming directly towards the observer does not seem to move at all, and marks the point from which all the others seem to radiate. 312. _The August Meteors._--A meteoric shower of no great brilliancy occurs annually about the 10th of August. The radiant point of this shower is in the constellation _Perseus_, and hence these meteors are often called the _Perseids_. The orbit of these meteoroids has been pretty accurately determined, and is shown in Fig. 353. [Illustration: Fig. 353.] It will be seen that the perihelion point of this orbit is at about the distance of the earth from the sun; so that the earth encounters the meteors once a year, and this takes place in the month of August. The orbit is a very eccentric ellipse, reaching far beyond Neptune. As the meteoric display is about equally brilliant every year, it seems probable that the meteoroids form a stream quite uniformly distributed throughout the whole orbit. It probably takes one of the meteoroids about a hundred and twenty-four years to pass around this orbit. [Illustration: Fig. 354.]