Radiant (meteor shower)
As a radian is defined as the point in the night sky, seem to take its origin from which meteor shower.
The drawing on the right shows schematically the distribution of meteor trails in the night sky. If one follows the luminous traces back, they intersect at point O, the radians. Since the intersection of Earth's orbit and meteor moves within the solar system only slightly, is fixed and the Radiant. The constellation in which it is located, is used as a term for the meteor stream. For instance, the radiant for the Perseids in the constellation Perseus.
Meteors in meteor showers flying parallel to each other, when they meet with the Earth's atmosphere. The retina of the eye is tiny compared to the sky, which is why she sees the luminous phenomena in a central projection. The parallel trajectories are perceived as converging lines, as well, the sun tends to surround radially as to the eye, parallel rays of the sun.
With increasing distance from radians increases the average length of the lighting tracks. Because these are the paths that the observer sees from the side, while the railways to move near the radians directly to him.