The Comets

The word "comet" inspires thoughts of bright, surreal objects with long tails. Actually, although twenty or so comets are discovered or recovered each year, it is quite rare for one to become visible to the naked eye, and even rarer for one to be impressive. Comet Halley, for instance, was quite bright during its 1985-86 apparition, and in March 1986 had a tail which more than filled the fields of binoculars. To the naked eye, however, it was just a fuzzy star. Comet 1996 B2 (Hyakutake) was much brighter than Halley, and from dark sites its tail stretched more than a third of the way from horizon to horizon from March 24 to March 26. Those in light-polluted urban areas, however, got at best a faint glimpse of the tail. Comet Hale-Bopp in 1997 put on a very good show. Although its head and tail were smaller than Hyakutake's, Hale-Bopp had a bright dust tail that could seen in urban and suburban skies. Those who saw both comets in dark skies almost universally favored Hyakutake, a testament to the destructive power of light pollution!

In any given year, several comets will become visible in small telescopes. The usual telescopic appearance of a 7th-9th magnitude comet is similar to that of an unresolved globular star cluster or a galaxy. This is why Charles Messier, in the 18th century, drew up his catalog of deep-sky objects--to prevent confusion of these with the new comets that he was always hunting. The observational method to distinguish a comet from other diffuse objects in the sky is to watch for motion: a comet's position among the stars will slowly change from night to night. Sometimes this motion can be noticed within an hour's viewing.

Because comets can brighten or fade rapidly, the Web is a good source for up-to-date information. Here are some of the best comet sites.

Some comets, like Halley's, are periodic, with orbits that take less than 200 years to complete. Others have orbital periods of millions of years. The orbits of some periodic comets cross the Earth's orbit. If there is dust strewn along the comet's orbit at the point where we cross it, we get a meteor shower. Meteors offer unique and intriguing observing opportunities.