The region of sky from which a meteor shower appears to emanate, as a result of perspective. In reality, meteoroids in a meteor stream pursue parallel orbits around the Sun, and enter the atmosphere along parallel trajectories. As an observational convenience, a radiant diameter of 8° is adopted. A meteor shower is named for the constellation in which its radiant lies (e.g. the Perseids emanate from Perseus). Because of the Earth’s orbital motion, a radiant appears to drift eastwards by about 1° each day. Photographic and telescopic studies can reveal true radiant diameters and a degree of structure; splitting of a meteor stream by planetary perturbations, for instance, can result in showers with multiple sub-radiants.