Estimating brightness is one of the most common activities for a comet watcher, but it can also be one of the trickiest. Among experienced observers, a popular procedure is the Sidgwick, or
First, fix in your mind the
"average" brightness of the comet's coma. Unfortunately this "average" tends to vary among observers. Then choose a comparison star, and take your telescope out of focus until the star reaches the size of the in-focus coma.
Compare the star's surface brightness with the memorized avenge brightness of the coma.
Repeat this procedure and try to find a star that matches the coma's brightness. You will usually find that the brightness is somewhere between the magnitudes of nearby stars.
Comets vary greatly in appearance. Observers often record how condensed the coma appears. A standard scale ranges from 0 (diffuse image, no condensation) to 9 (a bright, star-like image).
If the comet has a visible tail, you may like to note its length. A tail equal in length to the Moon's diameter is 30 arc-minutes long. Also record the direction in which the tail is pointing. As the comet cruises past Earth, the orientation of the tail can change rapidly over a few days.