Radiant (meteor shower)

The radiant or apparent radiant of a meteor shower is the point in the sky, from which (to a planetary observer) meteors appear to originate. The Perseids, for example, are meteors which appear to come from a point within the constellation of Perseus.

An observer might see such a meteor anywhere in the sky but the direction of motion, when traced back, will point to the radiant. A meteor that does not point back to the known radiant for a given shower is known as a "sporadic" and is not considered part of that shower.

Cause

Meteor showers are caused by the columns of dust and debris left in the path of a comet. This dust continues to move along the path of the comet, and when the Earth moves through such debris, a meteor shower results. Because the debris is all moving in roughly the same direction, the meteors which strike the atmosphere all "point" back to the direction of the comet's path.

Observation

The radiant is an important factor in observation. If the radiant point is at or below the horizon, then few if any meteors will be observed. This is because the earth shields the atmosphere from most of the debris, and only those meteors which happen to be travelling exactly (or very near) tangential to the earth's surface will be viewable.

References

* [http://eobglossary.gsfc.nasa.gov/Library/glossary.php3?xref=radiant Earth Observatory Glossary: Radiant] on NASA.gov