Variable Showers

Abstract

There are some meteor showers whose strength varies considerably from 1 year to the next. They may provide strong activity one year and none the next. Eleven of these showers are presented along with radiant charts for easy locating. Parameters such as activity period, date of expected maximum activity, and geocentric velocity are listed. A discussion of possible future displays is offered along with photographs of actual shower members obtained by the author.

There are meteor showers that only appear during years when circumstances are favorable. Most of these circumstances involve Earth crossing one of the dust paths created by the parent object. The showers listed here have the potential of producing meteor storms or no activity at all. For most of them the annual activity is closer to nothing at all. In several instances activity has appeared only once, and we await a possible reoccurrence sometime in the future.

Most of these showers are thought to be produced by debris produced by long-period comets during their previous return to the inner Solar System. Only rarely have the parent objects been discovered. These are the showers that normally only produce activity when Earth lies close to the dust trail. The other showers are produced by the Jupiter family of comets. These are comets with short orbits that lie wholly within the inner solar system and are infl uenced by passages close to the giant planet Jupiter. All of the showers on this list associated with the Jupiter family of comets have had their sources identifi ed.

An interesting fact about the following list is that a vast majority of the showers are best seen during the evening hours. This is a completely opposite situation when compared to the showers on the major annual shower list. It allows the meteor observer the opportunity to observe during the evening hours and possibly witness some activity other that the normal slow sporadic activity that usually occurs during this time of night.

Despite the normal low activity of these showers, some of them appear on the annual list of meteor showers in the hope that it will encourage observers to watch for them. The average returns of these showers are below the criteria set for minor showers, so identifi cation would normally be diffi cult for the visual observer. When observing during the evening hours, though, the sporadic rates are much lower than that seen during the morning hours; therefore, the chance of sporadic pollution of a dataset is much less. An observer can be confi dent that a meteor is a true shower member as long as the angular velocity and the path length are within the range given for each shower.

The variable showers are presented in chronological order, with a brief history, current status, and future prospects. Also included are the observing parameters listed— where are the best locations and when are the best times to view any possible activity. Although numerous outbursts have occurred in the past, those included here are predicted to be active in the twenty-fi rst century (Fig. 5.1).