Glossary

Absolute magnitude
The stellar magnitude any meteor would have if placed in the observer's zenith at a height of 100 km.
Aphelion distance
Abbreviation Q, distance of greatest heliocentric separation for a body in an eccentric orbit; Q=a(1+e).
Apollo asteroids
Asteroids having semimajor axes a>1.0 au, and perihelion distances q<1.017 au. See also: Asteroid, Aten asteroids, Perihelion distance, Semimajor axis
Asteroid
One of a number of objects ranging in size from sub-km to about 1000 km, most of which lie between the orbits of Mars and Jupiter; also called 'minor planet'. The preliminary designations consist of the year of discovery, an upper case letter to indicate the halfmonth in that year (A=Jan 1-15, B=Jan 16-31, ..., Y=Dec 16-31, the letter I being omitted), and a second upper case letter in sequence. When this sequence of 25 letters (with I again being omitted) has been completed it is repeated and followed by a sequential number. Permanent designations consist of numbers and names, beginning with (1) Ceres, given to asteroids for which orbits are accurately determined. Names are generally proposed by the discoverer. See also: Apollo asteroids, Aten asteroids, Meteoroid Stream
Aten asteroids
Asteroids having semimajor axes a<1.0 au, and aphelion distances Q>0.983 au. See also: Apollo asteroids, Asteroid, Semimajor axis
Comet
A diffuse body of solid particles and gas, which orbits the Sun. The orbit is usually highly elliptical or even parabolic. Comets are unstable bodies with masses of the order of 10^18 g whose average lifetime is about 100 perihelion passages. Periodic comets comprise only ~4% of all known comets. Periodic comets are designated by a number, followed by 'P/' and its name. E.g. Halley's comet has the designation 1P/Halley, the parent body of the Perseids, 109P/Swift-Tuttle. See also: Meteoroid Stream
Fireball
A bright meteor with an apparent visual magnitude of -4 mag. or brighter. See also: Meteor, Meteorite, Meteoroid, Persistent train
Geocentric
Earth-centered.
Heliocentric
Sun-centered.
Inclination
Abbreviation i., in the Solar System, the angle between an orbit and the plane of the Earth's orbit (ecliptic).
Limiting magnitude
Generally denotes the faintest star visible during an observation and evaluates the quality of the sky as well as the observing technique. The magnitude of the faintest meteor visible can be different from the stellar limiting magnitude, particularly for photographic and video observations. Visual observations assume about the same limiting magnitudes for stars and meteors. See also: Photographic observations, Video observations, Visual observations
Meteor
In particular, the light phenomenon which results from the entry into the Earth's atmosphere of a solid particle from space. See also: Fireball, Meteor Shower, Meteorite, Meteoroid
Meteor Shower
A number of meteors with approximately parallel trajectories. The meteors belonging to one shower appear to emanate from their radiant. See also: Meteor, Solar longitude, Trajectory
Meteorite
A natural object of extraterrestrial origin (meteoroid) that survives passage through the atmosphere and hits the ground. See also: Fireball, Meteor, Meteoroid, Micrometeorite
Meteoroid
A solid object moving in interplanetary space, of a size considerably smaller than a asteroid and considerably larger than an atom or molecule. See also: Fireball, Meteor, Meteorite, Meteoroid Stream
Meteoroid Stream
Stream of solid particles released from a parent body such as a comet or asteroid, moving on similar orbits. Various ejection directions and velocities for individual meteoroids cause the width of a stream and the gradual distribution of meteoroids over the entire average orbit. See also: Asteroid, Comet, Meteoroid
Micrometeorite
A small extraterrestrial particle that has survived entry into the Earth's atmosphere. The actual size is not rigorously constrained but is operationally defined by the collection procedure. Micrometeorites found on the Earth's surface are smaller than 1mm, those collected in the Stratosphere are rarely as large as 50 micro-m. See also: Meteorite
Path
The projection of the line of motion of the meteor on the celestial sphere, as seen by the observer.
Perihelion distance
abbreviation q, distance of the least heliocentric separation for a body in an eccentric orbit; q=a(1-e). See also: Apollo asteroids
Persistent train
Remaining glow due to ionization in the upper atmosphere after the passage of a meteoroid. The intensity and duration depend on the meteoroid's atmospheric entry velocity, its size, and its composition. Bright fireballs occasionally caused trains visible for several minutes. See also: Fireball
Photographic observations
The meteors are captured on a photographic film or plate. The accuracy of the derived meteor coordinates is very high. Normal-lens photography is restricted to meteors brighter than about +1mag. Multiple-station photography allows the determination of precise meteoroid orbits. See also: Limiting magnitude, Video observations
Poynting-Robertson effect
A dissipative force due to the anisotropic loss of momentum by a particle through re-radiation of solar energy. This causes aphelion collaps such that a circular orbit is soon attained; thereafter the particle spirals slowly towards the Sun. Small particles (below 1cm) are most severely affected because the force varies as the reciprocal of its size.
Radiant
The point where the backward projection of the meteor trajectory intersects the celestial sphere. More generally, the point in the sky where meteors from a specific shower seem to come from. See also: Trajectory
Radio observations
Two main methods are used, forward scatter observations and radar observations. The first are easy to carry out, but deliver only data on the general meteor activity; showers cannot be associated. The last is carried out by professional astronomers. Meteor radiants and meteoroid orbits can be determined.
Semimajor axis
Abbreviation a, half the length of the major axis of an ellipse, a standard element used to describe an elliptical orbit. See also: Apollo asteroids, Aten asteroids
Solar longitude
Angular distance along the Earth's orbit measured from the intersection of the ecliptic and the celestial equator where the Sun moves from south to north. It gives the position of the Earth on its orbit and, hence, is a more appropriate information on a meteor shower's maximum than the date. See also: Meteor Shower
Telescopic observations
Monitoring meteor activity by a telescope, preferably binoculars. This technique is used to determine radiant positions of major and minor showers, to study meteors much fainter than those seen in visual observations. See also: Visual observations
Trajectory
The line of motion of the meteor relative to the Earth, considered in three dimensions. See also: Meteor Shower, Radiant
Universal Time
The local mean time of the prime meridian. It is the same as Greenwich mean time, counted from 0 hour beginning at Greenwich mean midnight.
Video observations
This technique uses a video camera coupled with an image intensifier to record meteors. The positional accuracy is almost as high as that of photographic observations and the faintest meteor magnitudes are comparable to visual or telescopic observations depending on the used lens. Meteor shower activity as well as radiant positions can be determined. Multiple-station video observations allow the determination of meteoroid orbits. See also: Limiting magnitude, Photographic observations, Visual observations
Visual observations
Monitoring meteor activity by the naked eye. Least accurate method but easy to carry out. Large numbers of observations permit statistically significant results. Visual observations are used to monitor major meteor showers, sporadic activity and minor showers down to a ZHR of 2. See also: Limiting magnitude, Telescopic observations, Video observations, ZHR
ZHR
The number of shower meteors per hour one observer would see if his limiting magnitude is 6.5mag and the radiant is in his zenith. See also: Visual observations