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.
During the active period of most showers, the radiant moves nearly one degree eastwards, parallel to the ecliptic, against the stellar background each day. This is called the radiant’s diurnal drift, and is to a large degree due to the Earth’s own orbital motion around the Sun, which also proceeds at nearly one degree a day. As the radiant is determined by the superposition of the motions of Earth and meteoroid, the changing orbital direction of the Earth towards the east causes the radiant to move to the east as well.
Meteor showers are mostly caused by the trails of dust and debris left in the wake of a comet. This dust continues to move along the comet's wake, and when the Earth moves through such debris, a meteor shower results. Because all of the debris is moving in roughly the same direction, the meteors which strike the atmosphere all "point" back to the direction of the comet's path.
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 atmosphere shields the Earth 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.
- Earth Observatory Glossary: Radiant on NASA.gov
- Brian G. Marsden (1983-10-25). "IAUC 3881: 1983 TB AND THE GEMINID METEORS; 1983 SA; KR Aur". International Astronomical Union Circular. Retrieved 2011-07-05.
- "Exploding Clays Drive Geminids Sky Show?", 2010 October 12"Archived copy". Archived from the original on 2010-10-17. Retrieved 2011-01-18.