The best showers of the year are normally well advertised in advance through the various astronomy publications. As a result, many "seasonal" meteor observers join the veterans to watch the show. Whether you are a seasonal or veteran observer, most observers make some effort to record data - particularly during a major shower. Recording methods would vary from person to person if it wasn't for an attempt at standardizing the method. The International Meteor Organization was created to develop and promote meteor observing standards, and are the methods used by the North American Meteor Network. There are three reasons why all observers should follow these methods. First, they are straightforward and easy to learn. Second, the IMO represents observers worldwide, making it possible for all observations to be combined into one data set. And third, you've invested a considerable amount of personal effort into gathering your data - make sure it is accurate, useful and scientifically meaningful by adopting the IMO observing method.
There are two techniques that can be applied to visual observing. The first, known as visual counts, is the easiest for the beginner to learn. After sufficient experience has been gained, observers can begin using the second technique, that of plotting meteors.
The counting of meteors is by far the simplest method of observation. Basically, all that is required is for an observer to watch the sky and record each meteor observed. But, for your observations to be useful, certain data must be recorded for your reports. After investing your time in collecting all of the following information, it would be a shame if you did not make your observation useable by researchers.
The NAMN Visual Summary Report should be filled out as soon as possible after your observing session. The NAMN uses a form that can be submitted electronically or by postal mail. Appendix C contains a sample of this form. The filling out of the report is where most mistakes are made. I can't stress enough the importance of correctly filling out your report, therefore, please contact the Coordinator if you are having any difficulty with reporting your data.
Since meteor researchers rely on a large quantity of data from many observers, each person should record the same data using the same methods. The following discussion covers the information needed for the Visual Summary Report as well as the proper method of gathering it. Items are discussed as they appear on the report.
1) OBSERVER INFORMATION - The standard observer information such as name, address, date, universal time, longitude and latitude of your site, and recording method is required at the top of the form.
It is standard to use a double date. For example, the observing night of August 11/12 represents the evening of August 11 and the morning of August 12. Even if the observation began after midnight, the date is still written as August 11/12. All observations should be recorded in universal time. If you are unsure of how to convert local time to universal time, then it is best if the report shows local time which can be converted later by those reviewing the report. Just insure that whatever time you use, the type is recorded along with what time zone you are located in. With this information, your local time can be converted to universal time.
Your longitude and latitude should be listed in degrees, minutes and if possible seconds for your observing location. Be sure to include your elevation above sea level. This can be obtained from topographic maps available from the U.S. Geological Service or some bookstores. Road maps are not normally accurate enough for this purpose. If you need help in determining site coordinates, please contact the NAMN Coordinator.
The data recording method may be by tape recorder or manual (paper and pencil).
2) OBSERVED SHOWERS - During your planned observing session, one or more meteor showers may be active. If you plan on trying to keep track of multiple showers, attention must be paid to the proper assigning of each meteor you see. For those just starting out, it is usually easier to watch for one shower only, recording all other meteors as sporadics.
Somewhat difficult at first, classification of a meteor becomes easier with practice. When a meteor shower occurs, all shower members radiate from a central location known as the radiant. By knowing where a shower's radiant lies, a meteor can be projected backward along the path it traveled to see if it crosses the radiant. Radiant diameters vary depending on the shower. Most are in the neighborhood of 5 degrees in size, although some are as large as 15x10 degrees. If a suspected meteor is traced back toward this radiant, count it as a shower member. Otherwise it should be considered a sporadic meteor. Care should be taken on nights when more than one meteor shower is active. On these nights, all radiants in the area of sky that is being covered should be accurately identified prior to the watch.
A handy technique for determining if a meteor belongs to a specific shower is a 3 foot long piece of rope. Tie a loop in both ends. Slip your thumbs into each end and be ready for the next meteor. When it appears, immediately line this rope up with the meteor's path. If the rope lines up with any of your active radiants and the velocity looks appropriate (see below), then you most likely have a shower member.
There are three criteria used in determining if a sighted meteor belongs to a particular meteor shower or not. The three things to consider are:
a) RADIANT - First of all, the meteor must line up with a radiant. As mentioned previously, most meteor shower radiants are about 5 degrees in diameter. As a guide, keep in mind that 10 degrees is approximately that of a fist held out at arms length.
b) VELOCITY - Each meteor shower produces meteors that are all close to the same velocity. The velocity of a meteor is dependent not only on its own velocity, but also that of our planet as it moves through space. You can use a scale of 1 - 5 to assign velocity to a meteor. Very slow (visible for several seconds) would be assigned a 1, slow a 2, 3 for medium velocity meteors, 4 for a rather fast one with a meteor head visible and 5 for those moving very fast, just leaving streaks with no visible head.
c) METEOR LENGTH TO RADIANT RELATIONSHIP - The closer a meteor emerges from its radiant, the shorter in length the meteor will appear. If it appears right in the radiant, it will appear as a star suddenly brightening and then disappearing (known as point meteors). The further a meteor appears from the radiant the longer the path of the meteor normally appears. Associating meteors with a shower using meteor length is difficult at first, but with practice becomes second-nature. A useful rule to remember is one developed by the International Meteor Organization which states:
"For radiant elevations higher than 30° the apparent path length l of a shower meteor amounts at most to half the distance from the radiant to the start point. Consequently, the distance between radiant and start point of a plotted meteor on the chart has to be at least twice as long as the meteor path itself if the meteor is a suspected shower member."
When filling out the Visual Summary Report, list those showers that you were observing using their three-letter abbreviation. Also list the radiant location in right ascension and declination. The three-letter code and the radiant location at maximum can be obtained from the list of showers in Appendix A. Since sporadic meteors are considered random meteors, they do not need to be listed here. (Note: You will use the three-letter code Spo or SPO to identify sporadics later in the report.)
3) OBSERVING PERIOD - To make your observations worthwhile, observing periods should be a minimum of one hour or more in length. Keep in mind that if you take a break during a given observing period, you must subtract this time from the whole period. So, if you took a 10 minute break, you should make that observing period 70 minutes long so that you'll have 1 hour of "effective observing time," normally abbreviated as simply "Teff." Also, if you write your data down on paper as you observe, you must subtract that recording time from your total time observing (this is known as "dead time"). With practice you will be able to find an acceptable average, but as a guide use about 10 seconds per meteor to record all the pertinent data. Record your observing periods by universal time in this section.
The column "Field" is the center of your field of view. Although the typical field of view covers an area of sky of about 100º, where you look in the sky will affect your results. It's important to have some idea as to where you watched. Center your field of view to the south or southeast, or preferably between 20 and 40 degrees away from the radiant of an active shower. In addition, the center of your field of view should be in the range of 50 to 70 degrees above the horizon. At this elevation, you are able to see the area of sky from the horizon to the zenith, maximizing your chances of seeing meteors. The use of a lawn chair makes setting up at this angle a simple matter. Whatever location you choose as your field of the view, the center of it should be reported as a point in the sky using right ascension and declination coordinates. A handy technique to do this is to record a star or asterism near the center of your field of view. Then after the observing session is over, it is a simple matter to just look up the coordinates of this star or asterism and record them on the report.
As mentioned above, Teff is simply the total time during the observing session that you were actually watching the sky. Breaks and/or dead time are not included in the reported Teff. It is reported in decimal format such that a 60 minute observing session would be reported as Teff = 1.00. As a further example, a 72 minute observing session would be listed as Teff = 1.20. Note that for each 6 minutes of time, Teff = 0.10.
The NAMN, along with other observing groups, recognize the fact that the number of meteors an observer will see is reduced if any portion of their field of view is obstructed. To account for this, a correction factor is applied to all observations and is simply abbreviated as the letter "F."
Obstructions to a field of view can be caused by clouds, trees, buildings, or other objects. While observing, the percentage of sky that is blocked by these obstructions is noted at the beginning of the session and at any time a change has taken place. This percentage is noted on the Visual Summary Report.
Clouds present a difficult challenge to observers since they are constantly changing. It would be almost impossible to list every change in cloud cover that takes place. Therefore, it is recommended that if clouds are present, the average cloud percentage over the last 10 to 15 minutes be reported. This will prevent an observer from spending so much time counting cloud cover that no meteors could be seen!
Due to several factors, observations when cloud cover exceeds 20 percent are not recommended. Instead, take a break or wait for another night when the clouds do not prevent observations.
After the observation is over, the percentages listed on the report form (see Section 5 - OBSERVING FIELD OBSTRUCTION) can be converted to a decimal value by using an equation provided by the International Meteor Organization. The equation that is used is:
1
F = -----
1 - k
percent blockage x minutes
where k = ----------------------------
total observing period
As an example, on the night of August 11/12, assume an observing period of 0500 to 0630. Teff for the period was 1.50 (90 minutes), and the sky was covered with 15% clouds for 15 minutes.
Therefore,
15% x 15 minutes* 2.25
k = ----------------- = ---------- = 0.025
90 minutes 90
Then,
1 1 1
F = ----- = ----------- = --------- = 1.03
1 - k 1 - 0.025 0.975
***(When calculating k, be sure to include all individual cloud cover estimates in the percent x minutes value.)***
The correction factor F, equals 1.03 for the above mentioned observing period, and should be listed under the OBSERVING PERIODS section of the NAMN report. Note that when there are no obstructions to a field of view, F would then equal 1.00 and should also be listed for each observing period on the report. If you need help in recording or calculating this correction factor, contact the NAMN Coordinator.
The limiting magnitude of the sky (abbreviated as LM on the report form) determines the faintest star an observer can see. This will vary with the individual and is directly related to light pollution and/or the moon being above the horizon. It's determined by counting the number of stars within the boundaries of predetermined areas (see Appendix D). Usually one of these star count areas is near enough to the observer's center of view to make estimates accurate. After selecting the nearest star area, count all the stars you can see, including the corner stars. Later, you can determine the corresponding limiting magnitude from the appropriate table and record it on your report. Make these star counts at least every half hour, or more frequently when the sky appears to be changing. If your sky limiting magnitude drops below 5.0, either take a break or stop for the night. Corrections for a limiting magnitude this poor make observations unreliable. The LM you list on your report should be the average for that entire observing period (see Section 7 - LIMITING MAGNITUDE AND MEAN LIMITING MAGNITUDE).
Finally, note the number of meteors seen for each shower observed. Do this for the sporadics as well. Note that the number zero is used to indicate that you were watching that shower but none were seen. The slash character (/) indicates that during that period, you did not watch for meteors of that shower. This is normally done because the radiant of the shower is too low or because you were facing a direction of sky that prevented you from making reliable observations of that shower. As a check, make sure to fill out the totals columns for this section.
4) MAGNITUDE DISTRIBUTIONS - Magnitude distributions are obtained from the direct estimate of the magnitude of each meteor an observer sees. This is one of the most important contributions an observer can make to meteor science. Magnitude distributions are the first quantity determined in almost any research investigation. Therefore, always include a magnitude distribution for all meteor showers observed. This applies to sporadic meteors as well.
Everyone is probably familiar with magnitudes as it applies to astronomy. But when it comes to designating a magnitude for each meteor we see, difficulties can arise. Most people tend to overestimate their brightness when first recording magnitudes. Determining the magnitude of a meteor is done by comparing the brightness of the meteor to the brightness of a star with a known magnitude. This requires some knowledge of the locations and magnitudes of suitable comparison stars, but can be planned for prior to the observing session. To make comparisons accurate, you will need to identify a star of each magnitude between first and sixth. Also note the location and brightness of any brighter stars or planets that happen to be above the horizon. At the beginning of your observing session, take note of some stars for reference. Use the following list as a start:
(-12.0): full moon
(-8.0): quarter moon
(-6.0): crescent moon
(-4.0): Venus
(-2.0): Jupiter
(-1.5): Sirius
(-1.0): Canopus
(0.0): Vega, Arcturus, Rigel, Capella
(+1.0): Deneb, Altair, Pollux, Aldebaran, Spica
(+2.0): Polaris, Gamma Leonis, Alpha Andromeda, Gamma Geminorum, Alpha Ophiuchi
(+3.0): Beta Triangulum, Alpha Aquarii, Gamma Bootes, Epsilon Geminorum
(+4.0): Rho Leonis, Eta Persei, Delta Aurigae
(+5.0): Epsilon Lyrae
Check star charts and/or star catalogs beforehand to obtain magnitudes of other stars for reference during the night. Throughout the night, glance at stars with known magnitudes to acquire a feel for the brightness before a meteor suddenly appears. When it does, try to compare its magnitude to your chosen comparison stars, choosing the one that is closest in brightness. The magnitude of that star then becomes the magnitude of the observed meteor.
5) OBSERVING FIELD OBSTRUCTION - This is where the amount of your field covered by trees, buildings or clouds is recorded as a percentage. These percentages will be used after the observing session to calculate the value of F which is recorded under the OBSERVING PERIODS section of the Visual Summary Report.
As a reminder, estimate the percentage of field obstruction so that corrections can be applied to the data to account for meteors possibly missed. Clouds are difficult to estimate because they are constantly moving and are irregular in shape. But try to assign an accurate as possible percentage. If you reach a point in which 20 percent or more of the field is obstructed, you might as well take a break or just enjoy what you can see. Recording useful data at this point is impossible because adding the appropriate corrections to the data will make it unreliable.
6) DEAD TIME AND BREAKS - During a long observing session, breaks are a must. Recording the time that your attention is not focused on the sky for any reason is necessary as discussed in the section on OBSERVING PERIODS. Dead time is considered the time it takes you to record each meteor on paper and will be zero if using the tape recorder method of recording (assuming you have no dead batteries, tape failure, etc.).
7) LIMITING MAGNITUDE (LM) AND MEAN LIMITING MAGNITUDE - The average of the sky limiting magnitude for the entire night also needs to be reported. This requires little extra effort since the information was collected as explained in the OBSERVING PERIODS section above. On the report form, this is where each LM estimate is recorded along with the time, star count area used, and the results of the star count. List each estimate separately.
8) METEOR DATA - It is worthwhile to list each meteor you observed chronologically. For each meteor, several quantities are needed. First, record the time it appeared as well as the magnitude. The velocity is also recorded using the one to five scale previously discussed under Section 2b - VELOCITY.
The color of a meteor should also be reported on the form. Most meteors will appear white or yellow. Color can be an indication of meteor composition or the excitation of various atmospheric air molecules. Green is believed to represent oxygen molecules while blue has been suggested to indicate nitrogen. Brighter meteors often display a green or blue tint, although other colors are also reported. Some people try to assign colors to fainter classes of meteors, but there is probably not enough colored light to make the observation reliable. With anything fainter than +2.0 magnitude, it becomes increasingly difficult to assign color to a meteor.
The shower the meteor was a member of is listed using the shower association guidelines explained in Section 2 - OBSERVED SHOWERS. Meteors that are not a member of a recognized shower are listed as sporadics. For sporadics, use the three-letter abbreviation SPO.
If the meteor had a persistent train, the duration of the train is recorded. Many of the brighter meteors will leave behind this persistent train, which is simply a luminous streak of light that remains along the path of the meteor after it has passed. Usually, trains will fade rapidly and only last for a brief period of time. The typical duration of a train is in the order of a few seconds or less, although trains lasting 30 seconds or more are observed occasionally. The number of trains and their duration is valuable information for research purposes. If you can see a train for 0.5 seconds or more, record how long it lasted. If it's visible less than 0.5 second, it is referred to as a "wake." Not all meteors or meteor showers produce trains. However, some meteor showers do produce more trains than others. Among other things, train production is related to velocity. The faster the meteor enters the atmosphere, the greater the chance for meteor train production.
Historically, most observers throughout the world have recorded trains, but no central location was set up to receive the reports for train studies to be carried out. In most cases, data concerning persistent trains was either lost or collected on a regional basis only. The IMO in an effort to reverse this trend, has implemented a meteor train program where all train observations can be collected. It is hoped that this will provide a large enough database to allow more insights to be made about the phenomenon of train production. To participate in this study, a separate reporting form for meteor trains is required. To obtain a copy along with an instruction sheet, contact the NAMN Coordinator (see Appendix B).
The last two columns of the Meteor Data section of the report are Map and Accuracy. These apply to observations made using the plotting method and are discussed later in Section B of this chapter. When reporting observations using the visual count method, these columns are left blank on the NAMN Visual Summary Report.
9) SUGGESTIONS - In addition to the above, keep in mind major showers tend to bring groups of people together for observing, creating the problem of pooling data. Under no circumstances should this be done. If you prefer to observe with a group of people, insure that you record only the meteors you observed, not those pointed out by other observers. Finally, in preparing for a meteor observing night, probably the most important thing that you can do is not wear yourself out. Be sure to take a nap in the late afternoon or evening if at all possible. As the night goes on, you will find out how important this can be.
10) REPORTING - After reviewing your report for accuracy, send your reports immediately to the NAMN Coordinator (see Appendix B). Your data will be reviewed and forwarded to the International Meteor Organization.
If you are ready to go beyond counting meteors, plotting is the next step in meteor work. Plotting is especially well suited to the minor showers of the year where only a handful of meteors are observed each hour. An active shower that produces only a few meteors per hour is difficult to distinguish from the sporadic meteors. It's one thing to observe 20 or more meteors coming from a radiant and an entirely different prospect when only three come from the radiant. Are all three of these meteors shower members? Only one? Two? Plotting can help us answer this question.
In short, plotting consists of taking the path of an observed meteor and transferring it from the sky to a map or star chart. Special charts, known as gnomonic charts are used for this purpose. Plotting the paths of meteors should only be done when shower rates are below 20 per hour. When rates are higher, many meteors will be missed while you are busy making the plots. This results in unreliable data. Another point to keep in mind is that while staring at the sky (after a meteor passes) to get a good fix on its location, additional meteors may appear. When this happens, it probably is best to get an accurate plot of the first meteor while counting the others as sporadics.
The NAMN has adopted charts that the IMO uses, which are the BRNO atlas charts. These have been specifically designed for meteor plotting. If you try plotting a meteor onto an ordinary star chart, the meteor would have to appear curved. This is not how meteors travel, so gnomonic star charts are used where the constellations are stretched out near the edges. This allows meteor paths to be plotted as a straight line. Currently, a master set of these charts can be purchased from the IMO for $4.00.
To make a plot, simply hold a piece of rope to the sky aligned with the path of a meteor trail. While holding the rope in position, take a few moments to get your bearings in the sky by using known stars or star patterns. Once confident of the meteor's position, draw an arrow to match it onto one of the charts you have previously selected for the night. Label this arrow with a number that will correspond to the data that is also recorded on the NAMN form (time, magnitude, velocity, color, train duration, map number and plotting accuracy). This information is used to complete the Meteor Data section of the Visual Summary Report. Plotting accuracy is a number you assign to your plot based on how accurate you feel the plot for any given meteor was. One (1) is very accurate, two (2) is a normal plot and three (3) represents a rough guess. Generally, an accuracy of one will be meteors that appeared near the center of your view. Meteors rated as a three are usually those that were seen out of the corner of your eyes, or one you may be confused over the location of its path. After noting the accuracy, the map that your plot appears on is recorded. Each chart has a map number stenciled onto it. In your report of each meteor, it is extremely important that you list the map number of the chart that the meteor appears on. Otherwise, the person entering your data into the computer will have to hunt through all of your charts for each meteor!
After the observing session is over, you can look at your plots to determine shower association. Normally, it is best not to know the location of any active radiants prior to observing because these may tend to bias your observations. But once inside, determine the active radiant positions based on the list of showers in Appendix A. Then draw a small circle on your chart(s) and label each active shower to show its radiant. Make shower determinations based on:
1) the meteor's alignment with the radiant,Unlike when making visual counts with a tape recorder, plotting meteors WILL result in some time that you aren't actually watching the sky. And as with counting, this dead time must be accounted for. While counting using the manual recording method, dead time may have been about 10 seconds per meteor. But with plotting, it is going to take longer, somewhere around 30 seconds per meteor. If you are new at plotting, a more reasonable time may be something like 45 seconds per meteor. Total all of this dead time up. Keep in mind that breaks are not included here but are listed in the "Breaks" portion of the report.
When you complete the reports, send the NAMN Visual Summary Report to the NAMN Coordinator as done when reporting visual counts. But the charts have to be postal mailed to the Coordinator who will collect the plots and forward them to the IMO. On each of your charts, be sure to write your name and the date of the observation.
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