Most meteors seen in the course of an observing session are
faint ones. Only a small fraction exceed magnitude 0 which are
caused by millimeter-sized meteoroids. About one in 1,200
observed meteors becomes brighter than -5 mag, while only one in
12,000 reaches -8 mag (Rendtel and Knöfel, 1989). Bright meteors
are called fireballs. The definition of a fireball is somewhat
arbitrary and in the literature the required minimum magnitude
varies between about -2 mag to -6 mag. In the IMO Fireball Data
Center (FIDAC), we regard meteors of at least apparent magnitude
-3 mag (corrected for zenith position) as fireballs. By zenithal
magnitude we mean the brightness the meteor would have if it had
appeared in the zenith of the observing site. As an example, a
meteor appearing like -1 mag may actually be a fireball if it
moves only a few degrees above the horizon. The apparent
brightness decreases with the square of the distance between
object and observer, and furthermore, the absorption of the
light is proportional to