Although some 25 comets are found each year, most of these are much too faint to appear in small telescopes. However, several comets of 11th magnitude or brighter appear annually, usually when they are close to the Sun. Almost always discovered by amateur astronomers, you can often see these comets in small scopes when the sky is dark and moonless.
If the comet is brighter than 8th magnitude and the coma is condensed, it should be visible through 6 inch (150 mm) scopes. Dark skies are essential: a comet's ghostly light is easily swamped by a bright sky.
What makes comets so interesting to observe is their rapidly changing nature. A single comer can change radically in appearance as it swings past Earth, its geometry shifting so that we see different parts of the tail from night to night. It is also common to see the comet's coma change in shape and structure from week to week.
Occasionally, the ion tail of an active comet may appear to separate itself from the head. This "disconnection event" occurs when the comet's magnetic field reacts to changes in the solar wind. A new tail can be rebuilt in as little as a half hour, but the disconnection may recur every few days.
Less frequently, a comet will suddenly brighten by one or several magnitudes. This can be brought on by the onset of jet activity or by the ejection of a fragment from the nucleus. As Comet West rounded the Sun in 1976, the stresses of the Sun's gravity broke its nucleus into four pieces. This released great amounts of dust and gas, causing the comet to brighten from magnitude 0 up to -2, which is as bright as Jupiter although more spread out.