Radiant (meteor shower)
The vanishing point of parallel meteor paths in the sky.
The apparent radiant of a meteor shower is the point in the sky where, to an observer on Earth, the meteors seem to come from. The Perseids, for instance, appear to radiate from a spot inside the constellation Perseus. Although individual meteor paths show up at random places overhead, the paths of two or more meteors from the same shower will all trace back to this radiant. This happens because the meteors are actually moving along parallel lines in three-dimensional space; from the observer’s perspective, viewing a two-dimensional projection of the sky, these parallel lines converge at a vanishing point—the radiant. This geometric effect is the same as that seen in crepuscular rays, where parallel beams of sunlight appear to spread out from a single point.
If a meteor does not point back to the known radiant of a given shower, it is called a sporadic and does not belong to that shower. Shower meteors can sometimes appear a short time before the radiant has risen above the observer’s eastern horizon; in such cases, the radiant is still above the horizon at the meteor’s altitude. During most showers’ active periods, the radiant drifts eastward by nearly one degree each day, parallel to the ecliptic, against the background stars. This diurnal drift is largely caused by Earth’s own orbital motion around the Sun, which also proceeds at about one degree per day. Because the radiant results from the combined motions of Earth and the meteoroids, the eastward shift in Earth’s orbital direction pushes the radiant eastward as well.
**Cause** Meteor showers are mostly produced by dust and debris left behind in a comet’s wake. This material continues to travel along the comet’s path, and when Earth moves through it, a meteor shower occurs. Since all the debris moves in roughly the same direction, the meteors that enter the atmosphere all point back toward the comet’s trajectory. An exception is the Geminids, which are caused by the object 3200 Phaethon, thought to be a Palladian asteroid.
**Observation** The radiant is crucial for observing. If the radiant point is at or below the horizon, few or no meteors will be seen. This is because the atmosphere shields Earth from most debris; only those meteors traveling exactly (or very nearly) tangential to Earth’s surface are visible. The radiant points of several major annual meteor showers are listed below.
- Diurnal drift
- nearly one degree eastwards per day, parallel to the ecliptic
- Cause primary
- trails of dust and debris left in the wake of a comet
- Cause exception
- Geminids caused by object 3200 Phaethon, thought to be a Palladian asteroid
- Observation condition
- radiant at or below horizon results in few if any meteors observed
Lore & Background
The radiant is determined by the superposition of the motions of Earth and meteoroid. 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 diurnal drift is largely due to Earth's own orbital motion around the Sun, which also proceeds at nearly one degree a day; as the Earth's orbital direction changes towards the east, the radiant moves 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 Earth moves through such debris, a meteor shower results. Because all debris moves in roughly the same direction, the meteors that strike the atmosphere all point back to the direction of the comet's path. As an exception, the Geminids are caused by the object 3200 Phaethon, thought to be a Palladian asteroid.
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. Shower meteors may appear a short time before the radiant has risen in the observer's eastern sky; in such cases, the radiant is above the horizon at the meteor's altitude.
Reader's Guide
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. The radiant thus determines the optimal viewing window for a meteor shower, as observers must wait until the radiant rises above the horizon to see the full display. The concept also helps distinguish shower members from sporadic meteors: any meteor whose path does not point back to the known radiant is considered sporadic and not part of that shower. The geometric effect of the radiant—parallel lines appearing to diverge from a single point—is identical to crepuscular rays, where parallel sunbeams appear to diverge from the Sun. This understanding allows observers to identify which shower a meteor belongs to and to predict when and where to look for the most activity.
More in Meteor Showers 1-24
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