As for asteroids, there are many
physical quantities that can be measured to determine comets'
characteristics. Exactly as for asteroids, all these parameters
are not independent and measuring some of them means to be able
to deduce the others. All these quantities appear in a formula
that defines the apparent magnitude of the comet as a function of
the others characteristics (click
here to see the formula).
Geocentric and heliocentric distances
These quantities represent the comet's distance from the Earth and from the Sun in astronomical units (AU). Obviously, they change with time.
The relative magnitude of the comet represents its' observed brightness. This brightness must be obtained taking in account all the light emitted by the comet's coma as seen from Earth.
Because comets have size, measuring their brightness will appear less obvious than measuring a star's brightness (which appears as point-like). The relative magnitude of a comet is therefore normally obtained by comparing the comet's average surface brightness with that of defocused stars (matching the comet's size) of known brightness.
The brightness of the comet supposed at a distance of 1 AU from both the Earth and Sun (click here to know more about the general definition of relative and absolute magnitude). This quantity is normally estimated from the comet's light curve, and in contrast with asteroids' absolute magnitudes, it is far from being constant. In fact this value will change both during different phases of the comet's single orbit (pre- and post-perihelion) and, for what concerns periodic comets, from apparition to apparition.
The shape of a comet is not always easy to describe. In fact, the coma can be elongated or having a tail, and in this case, its diameter (usually given in minutes of arc ) represents the smallest dimension of the coma (usually at a right angle to the tail).
As well as asteroids, comets as well have observable light curves that show how they vary in brightness. As for asteroids , these curves represent the total integrated brightness (in this specific case of the coma) as a function of time or distance to the Sun.