Date. Except when quoting times in Universal Time (U.T.), a night must have a double date to avoid confusion. For example, August 11 / 12 means the evening of the 11th and the morning of the 12th.
Time. Time to the minute is adequate. Only observers working simultaneously from more than one station need to record time to the second, so that meteors observed jointly can be established. Radio time signals must be used to achieve this accuracy.
The kind of time used must always be noted, e.g. zone, standard, daylight, universal. This is not necessarily self-evident by inspecting the data forms, as some areas do not observe daylight or summer time.
Shower. The first letter of a major shower name should be indicated if at all possible. Minor showers can be noted as observing experience increases, also by use of minor shower lists (esp. Cook's). There is no need to indicate sporadic meteors, as these are understood to be sporadic from lack of any shower notation.
Color. The first letter of a color name should be given if one is seen. Meteors, that appear as "white" only should not be noted with "W". If no other color is seen then white is understood. Most faint meteors will appear white only because they are below the observer's color threshold. If more than one color is seen, the colors should be noted in the order of prominence. If white is a component color, it should be noted according to its prominence.
Magnitude. The magnitude of a meteor is noted by comparison with stars of known magnitude. Most observers are comfortable in making estimates to the nearest whole magnitude. More experienced observers can try half-mognitude estimates if they wish. But a serious problem also arises; bias towards the whole numbers. Every observer will show this bias to some extent, so that a graph of meteor totals vs. half-magnitude will show a sawtooth effect. The whole magnitudes will be the peaks on the graph and the half-magnitudes will be the valleys. Thus ability of observers to obtain reliable half-magnitude estimates is questionable.
Distance. The distance is the apparent path length of the meteors in degrees of arc. Distance is determined by comparison with well-known "yardsticks" in the sky, such as: the Pointers of the Big Dipper, Pollux and Castor, and Aquila's head (all 5 degrees), Orion's Belt (3 degrees), and the open side of the Big Dipper's bowl (l0 degrees). Many observers tend to exaggerate path length. As discussed in the section on radiant reduction, chronically obtaining path lengths over 10 degrees indicates overestimation. Bright meteors naturally have longer average path lengths than fainter meteors. Observing under less than ideal sky conditions, which eliminates most faint meteors, thus tends to increase average path length. Even so, there are enough short bright meteors to prevent any undue increase.
Distance has much less importance than most other meteor data. It is optional information which helps an observer to visualize what he saw upon looking over hls data sheet. Many observers choose not to record it at all, with no harm done.
Duration. Meteor duration is estimated to tenths of a second. This is the most difficult thing for beginners to gain proficiency on. A thousand or more estimations are required before reasonable results begin to appear. The problem is gaining a feel for fleeting phenomena and appreciation of the length of a full second of time.
Using a stopwatch is out of the question, with human reaction time amounting to some tenths of a second also. Observers have to make estimates by visual impression. Virtually all beginners initially estimate the duration of every meteor to be one or two seconds, which is impossible. The average meteor duration is more like 0.4 second. Following are durations and likely visual impressions. A streak or flash with no moving body visible is 0.2 to 0.3 second, which constitutes the majority of meteors. Rather rare is 0.1 second, only a few per year at most. The moving body may be visible at 0.4 second, and quite likely is at 0.5 second. At 0.7 second the meteor is rather prolonged and easy to see; definitely so at 1.0 second. Exceptions would be meteors with very long paths. The eye can comfortably follow a meteor lasting longer than 1.0 second, allowing time for brief study of the object.
There is no set upper duration for meteors. Experienced observers have reported an occasional meteor at ten seconds. Beyond perhaps five seconds, however, the possibility of re-entering space junk arises. The possibility must be considered in conjunction with path length and low altitude of a major shower radiant. A shower meteor skimming the top of the atmosphere might take 5 or more seconds to traverse 90 degrees or more, much too fast for a re-entry.
One method for studying the length of a second is to watch the second hand of a clock with a large face. A particular second can be selected, then meteors visuallzed as the second hand is crossing it. If the second is visibly subdivided into 0.2-second intervals short durations can be studied more easily. Watches frequently have these subdivisions.
Duration is not really as important as most other meteor data. Occasional observers should omit it. Duration can aid in determination of shower class, especially when the meteor could have come from more than one shower. The best example is separation of slow Alpha Capricornids from swift Delta Aquarids, both of which are active in July and August. In addition, showers of slow meteors are much easier to distinguish from sporadics.
Train Duration. Some meteors leave behind a glowing streak, or train, along their paths. A very important piece of information is the duration of the train, timed to the nearest 0.5 second. The majority last no more than 2 seconds. For longer trains a stopwatch can be used, especially if they are followed in binoculars. Without a stopwatch, in truth, one probably cannot obtain 0.5 second accuracy beyond 3.0 seconds. There are only a few trains exceeding 3.0 seconds so this handicap does not really matter.
Train durations are used for deriving relationships with magnitude. The general trend is for brighter meteors to leave longer trains. Durotions can also be linked with true meteor velocity in many cases. Thus the major showers, which have various velocities, generally also have characteristic percentages of meteor trains.
Meteor trains enduring longer than ten seconds usually drift and distort. The direction of drift and notes on the distortion should be carefully recorded. Plotting the train at one-minute intervals is recommended if possible. So much detail passes rapidly as a train distorts that plotting is rendered very difficult. Trains sometimes last over an hour.
For both number and duration of trains, the Leonids are most famous, followed by the Perseids.
Altitude. The apparent altitude of a meteor's endpoint in degrees above the horizon can be recorded in an optional program. Alternatively, observers with obstructed or mountainous horizons can use zenith distance (degrees from the zenith). One should specify which is being used. Often observational forms will call for zenith distance, but individuals may use altitude if they wish, providing the form's column heading has been suitably modified
The purpose of altitude data is to correct meteor magnitudes for extinction. The correction formula is 5 log secant Z, where Z is zenith distance. A table of resulting corrections from the formula appears below. Once can see that one-degree accurocy is not needed except for very low altitudes.
If one is recording information on individual meteors, magnitude corrections can be made after the fact. Some observers even make no attempts to correct their magnitudes, but indicate only raw values in their tables.
If one is simply recording the number of meteors within each magnitude interval then some means of estimating zenith distance corrections is desired. It is suggested that at least the six highest altitude brackets be memorized, and that some mnemonic recording system be used for these. One idea is to use the round numbers 90, 50, 30, 25, and 20 degrees to represent their respective brackets. Most such simple schemes will handle a great majority of meteors.
When using zenith distance it is imperative that the zenith be accurately located. This cannot be done by just trying to look straight up - a ten degree error by this method is common. Using plot charts to find stars on the declination circle passing overhead, one can then keep track of the zenith as time passes. Observing to the south has a great advantoge: charts can be used to check altitudes easily for meteors close to the meridian, resulting in considerable accuracy. Most of the time the proper bracket can be decided upon without chart help; when a meteor ends near the border between two altitude brackets, the chart inspection resolves the conflict. Facing other directions, except north, puts an observer on his own for figuring altitudes.
Statistical methods are available to combine whole-magnitude data with half- magnitude extinction correction. In any event, the observers should always note whether the reported magnitudes have had extinction corrections applied or not.
Altitude MAG. Altitude MAG. (degrees) CORRECTION (degrees) CORRECTION 64-90 0 9-1O -4.0 46-63 -0.5 7-8 -4.5 35-45 -1.0 6 -5.0 27-34 -1.5 5 -5.5 21-26 -2.0 4 -6.0 17-20 -2.5 3 -6.5 13-16 -3.0 2 -7.5 11-12 -3.5 1 -9.0
Distance from Central Vision. This is another new, optional program. The observer notes, to ten-degree accuracy only, how far from his central vision he first saw each meteor. Once again, beginners had best wait a while before trying this. The DCV program is another attempt to evaluate differences in observer rates under uniform conditions, but without requiring observers to watch the same small area of sky or keeping their eyes still. It is hoped that some simple relation will be found between average DCV and average observed rate.
Limited results thus far show the expected trend of higher rates coming from larger average DCV's.
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