Great comet

A great comet is a comet that becomes exceptionally bright and easily observable to the naked eye. There is no official definition; the term is often attached to comets such as Halley's Comet, which, during certain appearances, are bright enough to be noticed by casual observers not looking for them and become well known outside the astronomical community. Typically, they are as bright or brighter than a second-magnitude star and have tails that are 10 degrees or longer under dark skies. Great comets appear at irregular, unpredictable intervals, on average about once per decade. Although comets are officially named after their discoverers, great comets are sometimes also referred to by the year in which they appeared great, by using the formulation "The Great Comet of ...", followed by the year. The term can also be used as a generic name when a very bright comet is discovered by many observers simultaneously.