Familiarity with stars to magnitude 4.0 is essential for accurate plotting. The beginning plotter might have to observe several hundred meteors before his plotting shows reliable results. Practice at fairly regular intervals is needed for maintaining plotting skill. Observers who work only a few nights a year should not expect to stay in practice for plotting. The occasional observer should instead concentrate on filling in the data forms. The radiants of the major showers are so well known there is little value in plotting them except for practice. Experience plotters might try "re-discovering" the major showers every few years. Beginning plotters can use major showers for training. Shower members of magnitude -3 or brighter should be plotted for possible height and orbit determination, in case another observer elsewhere also saw them.
The major function of plotting is to find minor radiants. All meteors well seen that do not belong to major showers should be plotted. This is a good program to carry on any time, even during major showers. The directions east to south are usually best as radiants concentrate around the ecliptic in the latter half of the night. Northern observers can extend coverage northward by facing northeast. North to west are the least productive directions.
The actual plotting effort involves getting the meteor's path onto a chart as accurately as possible. In drawing the path a ruler must always be used. It can also be held up ogainst the sky to establish the path location. It is suggested that both eyes be kept open for this operation, for the ruler can still be seen against the light sky yet not occulting stars.
The easiest plot results when a meteor begins and ends at two stars. Next easiest is a star marking beginning or ending, with direction towards another star. A path wholly between two stars is not too difficult. For harder paths fractions of distances between stars for beginning points and endpoints can be estimated, or two stars that form a line parallel to the path a known distance away can be used. Occasionally a meteor does no fit any of the above categories, so the observer just has to do the best he can.
Plotting against rich star backgrounds, usually Milky Way constellations, is naturally much easier than plotting against star-poor regions. Meteors that leave trains are easier to plot since the train allows prolonged observation of the path.
CHART USAGE. Plotting during a two- to four-hour session should be concentrated on no more than four charts. Using just two charts iis even better. Radiants are more likely to show up if many meteors are plotted on few charts. Thus, more hours of observing on one night are more apt to produce radiants. Projecting paths from one chart to another with accuracy is difficult, and projection to a third chart is all but impossible. Choice of direction for observing and observer latitude do much to determine the number of charts needed.
Charts for one night with at least ten plots should be replaced with new charts, unless the supply is short. When necessary to use the same chart again a different color pencil should be used. Single plots of meteors fainter than magnitude -2 rarely have value. A complete set of charts can be kept on hand for plotting casual meteors and especially fireballs.
RADIANT REDUCTION. A radiant requires an absolute minimum of three meteors for determination. But the probability of sporadic meteors projecting through such a radiant by chance is rather high. So a minimum of four meteor projections intersecting in a circle two degsees across is a safer requirement, increasing the probability that a real radiant may have been found. Chance intersection of sporadics in a radiant decreases rapidly with more meteors involved. A radiant can also be obtained from two successive nights, with three meteors on one night and two on the other.
Path length is quite important as it gives some clue to radiant location. Short meteors are expected near a radiant; long meteors far away. Among observers not plotting lengths well, for some reason more tend to plot paths too long rather than too short. It is hard to determine an average path length as different observers will get different results, but observers that never see meteors shorter than ten degrees long are most certainly exaggerating distances. Stationary meteors reveal their radiant directly and should be noted carefully.
Sharp radiants containing four meteors are uncommon, and five or more meteors projecting back to a single point are extremely rare. Observers who tend to obtain such radiants frequently may be biased with a preconceived notion of the radiant location. Radiants located at bright stars can be occasionally expected by random distribution. A frequent an occurrence of this type might mean an observer is affected with eye flashes caused by stars, particularly if the objects are not seen well. Or the observer might be attempting to define the radiant mentally, than biasing his plots accordingly.
Radiants as immediately determined from charts are more properly called "intersections", as their reality has not been established. Confirmations ot a given radiant from either past or future observations can be taken as evidence for reality. A radiant confirmation is another radiant which has nearly the same location on nearly the same date in a different year.
When multiple radiants appear on a single chart, meteors frequently project through more than one radiant with no certain way of deciding to which radiant they belong. In these cases the certain minimum and possible maximum number of meteors belonglng to each radiant are listed. For example, "3-5" for the "number of meteors" entry in a radiant table means that 3 meteors were in that radiant exclusively, while 2 others may or may not belong. A large number of such radiants on one night of observing fs unresolvable confusion. Confirmations are then needed for more definite determinations.
Letter quality ratings are given for radiants: A - excellent; B - good; C - fair; D - poor. Criteria for judging quality are sharpness, the total number of meteors lnvolved, amount of entanglement with other radiants, confirmations, amount of divergence in the plot paths, and number of nights used. A radiant determination over two nlghts is considered less favorable than one.
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