Theradiant orapparent radiant of ameteor shower is thecelestial point in the sky from which (from the point of view of a terrestrial observer) the paths ofmeteors appear to originate.[1] ThePerseids, for example, are meteors which appear to come from a point within the constellation ofPerseus.
Meteor paths appear at random locations in the sky, but the apparent paths of two or more meteors from the same shower will diverge from the radiant. The radiant is thevanishing point of the meteor paths, which are parallel lines in three-dimensional space, as seen from the perspective of the observer, who views a two-dimensional projection against the sky. The geometric effect is identical tocrepuscular rays, where parallel sunbeams appear to diverge.
A meteor that does not point back to the known radiant for a given shower is known as asporadic and is not considered part of that shower.
Shower meteors may appear a short time before the radiant has risen in the observer's eastern sky. The radiant in such cases is above the horizon at the meteor's altitude.
During the active period of most showers, the radiant moves nearly one degree eastwards, parallel to theecliptic, 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 ofdust and debris left in the wake of acomet. This dust continues to move along the comet's wake, and when theEarth moves through suchdebris, 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.
As an exception, theGeminids are a shower caused by the object3200 Phaethon,[2] which is thought to be aPalladian asteroid.[3]
The radiant is an important factor in observation. If the radiant point is at or below thehorizon, 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.
Here are the radiant points of some major meteor showers of the year.
| Meteor Shower | Radiant Point |
|---|---|
| Quadrantids | Constellation Boötes, near the former constellation Quadrans Muralis |
| Alpha Centaurids | Constellation Centaurus, near the star Alpha Centauri |
| Lyrids | Constellation Lyra, near the star Vega |
| Eta Aquariids | Constellation Aquarius, near the star Eta Aquarii |
| Arietids | Constellation Aries |
| Southern Delta Aquariids | Constellation Aquarius, near the star Delta Aquarii |
| Alpha Capricornids | Constellation Capricornus |
| Perseids | Constellation Perseus |
| Aurigids | Constellation Auriga, near the star Capella |
| Draconids | Constellation Draco |
| Orionids | Constellation Orion, near the star Betelgeuse |
| Southern Taurids | Constellation Taurus, near the star Aldebaran |
| Northern Taurids | Constellation Taurus |
| Leonids | Constellation Leo, near the star Regulus |
| Geminids | Constellation Gemini |
| Ursids | Constellation Ursa Minor, near the star Kochab |
| By name |
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|---|---|
| By peak date | 
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