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2017 Jan 15 15:49 UTC

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Meteor Glossary

  • Bolide. A term sometimes used to describe bright meteors which break up.
  • Curved Meteors. Meteors whose paths are not straight. In some cases it may be that by chance a second meteor appears at the point where another meteor has just ended. Other explanations involve the meteor being a bat or night flying bird lit up by streetlights. Unfortunately visual observations of curved paths will always be suspect: a satellite followed against the star background will appear to wander slightly from side to side!
  • Dark meteors. These are meteors which appear darker than the sky background. Although some could be meteoric in origin, other possible explanations involve floaters in the eye and unilluminated objects (e.g. birds, insects) being seen against the less dark sky background.
  • Daylight Showers.These are meteor showers whose radiants are only above the horizon during the hours of twilight or daylight and so can only be studied using radio methods. Several are active during May and June, including the Omicron Cetids and Beta Taurids.
  • Diurnal Variation. This is the tendency for meteor rates from the sporadic background to be significantly higher late in the night than early in the night. It is due to the fact that before midnight an observer is looking in the direction from which the Earth is travelling (and so some meteors are being left behind) whereas after midnight the observer is on the side of the Earth facing the direction of travel (and the Earth is 93catching up94 with some meteors).
  • Fireball. Although occasionally used for any meteor of negative magnitude, this term is usually reserved for much brighter meteors. The International Meteor Organisation defines fireballs as being meteors of magnitude -3 or brighter. The meteor section of the British Astronomical Association on the other hand has a much stricter definition, requiring the meteor to be magnitude -5 or brighter.
  • Flare. The sudden brightening of a meteor, most commonly near the end of its path when it is referred to as a Terminal Flare.
  • Limiting Magnitude. A measure of the darkness of the sky during a meteor watch. It is defined as the magnitude of the faintest star visible in the area of sky under observation
  • Meteor Storm. The rare occasion (a few times per century) when the sky is filled with meteors. It is usually defined as when the meteor rate exceeds 1000 per minute.
  • Minor Shower. These are the lower activity - usually defined as ZHR less than 10 - meteor showers. Unfortunately they can be difficult to identify reliably using visual methods and consequently lists of minor showers are usually littered with many spurious showers.
  • Nebulous Meteor. Meteors that appear nebulous rather than being sharply-defined are occasionally reported. Various explanations have been suggested, ranging from very fragmentary meteors, through night flying birds and bats, to ball lightning.
  • Path. This is the route of the meteor through the atmosphere when projected against the star background. (cf trajectory)
  • Poynting-Robertson Effect. This is an effect of solar radiation which affects low mass particles more than higher mass particles. It changes the orbits of the particles slightly. A result of this is seen in some showers for which the peak rates of faint (low mass) meteors occur earlier than the peak rates of brighter meteors.
  • Radiant. This is the area of sky from which the meteors of a particular meteor shower, when their paths are projected backwards, appear to have radiated. It marks the direction in space from which the meteor stream is approaching the Earth. Although meteor shower lists often give the impression that the radiant is a point in the sky, it is in practice an area of sky a few degrees across.
  • Seasonal Variation. This is the tendency for sporadic meteor rates to be significantly higher in the autumn than in the spring for northern hemisphere observers (and the reverse for southern hemisphere observers). The reason is that the direction in which the Earth is travelling is higher in the sky in the autumn than in the spring (e.g. Gemini in September Sagittarius in March).
  • Sounds. Sonic effects are sometimes reported from bright fireballs minutes after the visual sighting. However, some observers sometimes report sounds from more ordinary meteors simultaneous with the appearance of the meteor. Descriptions include swishing sounds, cracks, hissing. Although some observers believe the sounds to be genuine, possibly involving some (unspecified!) part of the electromagnetic spectrum, the prevailing view is that the sounds reported are purely psychological - an analogy with fireworks.
  • Sporadic Meteor This is a meteor which does not belong to a meteor shower. In practice, the sporadic meteor background will include meteors from unidentified minor showers.
  • Spurious Meteors. These are objects seen during meteor watches that appear similar to meteors but aren't. Examples include flashing satellites and planes, twinkling bright stars near the edge of the field of view and birds illuminated by streetlights. Other cases may be due to flashes of light within the eye-brain system.
  • Spurious Showers. If the paths of two meteors are projected backwards there is a 50% chance that they will intersect somewhere in the sky. By chance, other the paths of other unrelated meteors may also project back into this area of sky and the discovery of a new meteor shower is claimed. The higher the sporadic rates, the greater the likelihood that such chance lining-ups will occur and a few misidentified major shower meteors can also be included.
  • Stationary Meteor. A meteor that is travelling head-on towards the observer and thus appears as a flash of light at a point in the sky.
  • Trail. An ambiguous term - it is better to use either path or train.
  • Train. The ionised dust and gas along the path of the meteor which remains visible for a number of seconds after the meteor has passed.
  • Trajectory. This is the actual route of the meteor through the atmosphere. (cf path)
  • Wake. A meteor train that is so short lived that it appears to move along with the meteor.
  • ZHR The Zenithal Hourly Rate of a meteor shower is the number of meteors from that shower that a single observer with an unobstructed view would see in one hour if the meteor shower radiant was directly overhead and the Limiting Magnitude was 6.5. Under less favourable observing conditions, the number of shower meteors seen is likely to be somewhat lower.

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