A guide to terms used in the 
Canadian Fireball database


 

Date/time: The date is that which corresponds to the universal time. Universal time is 4 hours later than Atlantic Standard Time. 5 hours later than Eastern Standard Time, etc.

Observer Location is specified with latitude (north) and longitude (west).

The beginning and ending points of the meteor are specified in both earth based and celestial coordinates . In the case of earth based coordinates we use the angle from the horizon up as elevation (e. g. 9O degrees wouid be directly overhead while 0 degrees wouid be on the horizon). The bearing is an angle measured from north through east - e.g. southeast would be a bearing of 1 35° (note: some European countries measure bearing in the opposite direction of rotation). Note that these are "true" north directions, not compass north. If compass readings are used the magnetic offset correction should be applied before the coordinates are recorded. The celestial coordinate system is specified in terms of declination and right ascension. These are angular measures like latitude and longitude respectively, but refer to positions on an imaginary celestial sphere. The extensions of the celestial poles are declination + and - 9O°. Except for negligible "proper motion" and the slowly changing effects of precession of the Earth's polar direction, the stellar positions are "fixed" in a right ascension and declinations system. Therefore these coordinates can be used to test various possible "radiants" for any given meteor (to check for shower or group association). One can use desktop planetarium programs to readily convert from celestial to earth based coordinates, or vice versa. A useful technique for roughly estimating angles is to use the approximation that the fist on the outstretched arm is about 1O°. We note the apparent orientation of the trail to the horizon as well. The report notes whether the ending point was a true ending point (i.e. luminosity ended), or whether the end was blocked by a local horizon. We also note the cloud cover in %.

The astronomical magnitude scale is used in specifying the brightness of a fireball. This scale, which goes back thousands of years in its origin but now has a precise mathematical basis, is a logarithmic intensity scale, with 5 magnitudes meaning exactly a factor of 100 in brightness. Also, lower values mean brighter objects. For example, a -9M meteor gives off 100 times as much light as a -4M meteor. The stars in the "big dipper" (Ursa Major) are about + 2 magnitude, the brighter stars in the sky are a bit brighter than -1 magnitude, the planet Venus has an apparent brightness of about -4 magnitude, the full moon has a magnitude of -12.4 and the sun has an apparent magnitude of almost -27M. The dark adjusted human eye has a limit of about +6.5M in a good location. The limit of the military detectors is about -1 8M for fireballs. The number of meteors goes down inversely with increasing mass, so very bright fireballs are rare. We note a magnitude estimate when possible, and in all cases state whether the object is brighter than, about the same as, or fainter than the full moon.

It is important to differentiate the meteor trail (more or less instantaneous brightness) from the train (persistent luminosity which lasts seconds or longer). The duration of the meteor means the time while the trail was bright, and ranges from about 1/3 second to 40 seconds in extreme cases of near horizontal entry. The train duration (if any) is separately noted.

The colour of the fireball is noted. The human eye loses colour vision before total vision, which is why all faint meteors appear white. Bright fireballs display a range of colours throughout the spectrum. One often has atmospheric reddening effects of meteors which are low to the horizon.

The speed of a meteor is an important criterion in whether a meteorite will result. When possible we take the flight path (in degrees) and divide by the duration (in seconds) to obtain an approximate angular speed. In other cases a slow/medium/fast qualitative measure is used.

Bright fireballs frequently fragment under the combined influences of thermal stresses, shock waves, and differential aerodynamic pressure. We note the fragmentation, if any, with a qualitative description. Related to that, is the occurrence of "flares" or sudden brightening of the meteor trail (supposedly produced by exposure of increased surface area by fragmentation).

Sounds are of two types: near instantaneous electrophonic sounds, which come from electromagnetic waves converted into audible sounds by nearby objects; and anomalous sounds which are delayed seconds or minutes, often sound like rumbling thunder, and are a sign of penetration into the lower atmosphere. In both cases a description of the sound, and the delay time is noted.

We apply a system for noting the precision of the report, particularly with respect to end points of the fireball.The quality codes are assigned on the following basis. Indicated in brackets are the probable errors (expressed in degrees) in angular measurements.
 
 
F Very questionable accuracy; no estimates of directions [>4O]
D Rather weak and uncertain memory of directions - vague [2O-4O]
C Standard visual observation - rough indication of directions based upon assumed directions of local objects [5-2O]
B Visual observation, where care was made to recheck directions using compass (with correction for magnetic offset) or topagraphic map information, or where the original observation was specified relative to well determined stellar or lunar locations. [O.5 to 5]
A Instrumental observations (e.g. photographic) [<O.5]

We assign the code predominantly on the basis of error in angular measurements (and not on the basis of magnitude or time information).



Text by Bob Hawkes and Damien Lemay (Royal Astronomical Society of Canada)