How to Estimate a Comet's Brightness
Judging the magnitude of a comet is not straightforward.
Stellar magnitude estimates made by long-time variable-star observers often agree to within 0.1 or 0.2 magnitude. These observers are comparing stars with stars. But a comet's coma or head may be anywhere from a few arcminutes to a degree or more in size. Because comets appear radically different from the pointlike stars used for brightness comparisons, determining a comet's integrated (total) magnitude is far more difficult.
For centuries the reported magnitudes of naked-eye comets were very ambiguous. Often they seem to refer to the brightness of the intense nuclear condensation the strong, sometimes starlike feature seen at the heart of the coma. As such, the total brightness of the comet's head was usually underestimated. Not until the turn of the century were satisfactory visual methods developed for determining the brightness of extended objects.
As with variable stars, ascertaining a comet's brightness requires two comparison stars of known magnitude one slightly brighter than the comet and the other slightly fainter. It helps greatly if they are all in the same field of view and at a similar altitude above the horizon to avoid errors caused by atmospheric extinction.
Listed on the next page are five widely recognized methods used by amateurs to estimate a comet's integrated brightness. Each has its faults, but all (except perhaps the last one) will give acceptably accurate results if carefully employed.