Frequently Asked Questions on Visual Observations
- I could hardly distinguish the Aquariids or the Taurids. How to note this on the observing form?
- Which is the best way to determine a reliable value for the limiting magnitude?
- About 25 per cent of the sky were covered by houses and trees. Does it affect the cloud factor?
- How do I correctly estimate the field correction factor?
- Which items are of particular importance to record when plotting meteors?
- How do I get the Atlas Brno 2000.0?
- How do I measure meteor coordinates on the Atlas Brno? The origin is ambiguous.
- How reliable is the velocity estimate in degrees per second?
- How to treat meteors which could not be associated with a meteor shower unambiguously?
- Where do I send my visual observation results?
- How should the observing fields be distributed over the sky when observing in a group?
I could hardly distinguish the Aquariids or the Taurids. How to note this on the observing form?
Associating a meteor with a double or multiple radiant seems to be possible only from observing fields near the radiants. There is no chance to resolve the showers directly under the sky. The human mind turns out to be weak when constructing great circles onto the sky. You may test this if you see a satellite, which moves on a great circle definitely. Try to predict his location 40 degrees ahead and you watch the satellite’s motion. It will probably move several degrees off your prediction. 40° is about the distance between Deneb (alpha Cyg) and Altair (alpha Aql) or between Regulus (alpha Leo) and Procyon (alpha CMi).
If you do not watch the shower complex direct you should not try to distinguish the shower meteors, unless you make plottings. Although, meteor plots are rather uncertain graphs of the meteor’s path, you can associate the shower members by objective criteria after the observation. Meteor plottings are in fact urgently needed to follow the branches of the multiple radiants.
Please do not report meteor numbers of single branches besides sumarized numbers of the complex. If once an analysis searches for a certain branch, the meteor numbers stored as belonging to the branch will not represent the complete activity as there will be several meteors stored as belonging to the whole complex. Hence, if there is a combined shower besides its branches on your observing form all branch meteors will be put into the combined shower class. In the case of the Taurids either note the Northern and Southern Taurids or the Taurids in total. If you simply count the meteors during the Perseids you may report the Aquariids and Capricornids only.
Which is the best way to determine a reliable value for the limiting magnitude?
It turned out that a very unprejudiced and convenient method to dermine the limiting magnitude is to count the number of visible stars in certain areas on the sky. There are 27 regions all over both the northern and southern hemisphere. The more stars you can see the fainter is the limiting magnitude. The tables to obtain the limiting magnitude from the numbers of stars are given in the instructions for major shower observations
Some limiting magnitude areas bear systematic problems. If the observer has sensitive eyes but cannot resolve close stars very well, he will probably encounter difficulties with area 14 in Cygnus. It is recommended to these observers not to use this area when the limiting magnitude is better than 6.2m (i.e. more than 16 stars would be visible).
Area 1 in Draco has a significant gap in the limiting magnitude levels. It jumps from 5.3m to 6.0m between 10 and 11 visible stars. If you see 10 stars it is not clear whether the limiting magnitude was 5.3m, 5.5m or even 5.9m. Hence, switch to another area when you see just 10 stars in area 1. The same problem occurs with area 12 in Serpens/Libra. It jumps from 5.8m to 6.4m between 13 and 14 stars.
Since there are a lot of smaller gaps in other regions it is very obvious, that a reliable value of the limiting magnitude can only be obtained when you count the star numbers in more than one area. Moreover, you might sometimes not be sure whether a star is still inside or just outside the area. This effect adds a statistical uncertainty to the limiting magnitude estimate. The following items should be regarded when estimating the limiting magnitude:
- Use 2 or 3 areas where you determine the star numbers, the choice of the areas may change during the night.
- Average the limiting magnitudes of these areas to obtain a mean limiting magnitude.
- Determine the star numbers about every 0.5 h or 0.75 h.
- Do not use areas being very low at the horizon (<40° elevation), neither it is recommended to observe there at all.
- Different observers will have different limiting magnitudes; every observer should determine his own limiting magnitude.
- An accuracy of +-0.5m of the limiting magnitude is not sufficient for a serious analysis of your data.
About 25 per cent of the sky were covered by houses and trees. Does it affect the cloud factor?
The answer is simple: no. The field of view has a diameter of well 100° . The number of meteors being detected outside this field is very low against the number of meteors in the field. If you find a part of the sky to watch in which has a diameter of 100° and it is not covered by clouds or terrestrial objects, your cloud factor will be 1.0 independently of the clouds of any size around your field.
Some people observe from balkonies. They only see half of the celestial sphere at best. Although the diameter of the observing field does not exceed 90° between the horizon and the roof, the horizontal width of the field is certainly larger than 100° . The error is really very small if we set the cloud factor (more general: the field obstruction correction) to 1.0 also in these cases. I have watched several years from my balkony and did not notice a loss of activity which needs to be corrected.
How do I correctly estimate the field correction factor?
It often happens that just the interesting parts of a shower’s activity period are affected by clouds passing through. Unfortunately, the correction factor is the most difficult to determine. Some items should help you to correctly estimate the cloud correction.
The field correction does not include the entire sky. As explained in another FAQ the diameter of the effective field of viewis not larger than about 100° . Very few meteors are detected outside this area. If clouds appear you will naturally turn your head towards the raiming clear parts of the sky. Hence, the cloud factor still remains 1.0 unless the visible cloud gap is smaller than 100° .
Let us now consider clouds which cover more and more parts of the sky. When the usable field of view between the clouds becomes smaller than 100° we will note a cloud coverage. But be careful. You are probably centering your eyes into the cloud gap; this is the natural reaction on a restricted field of view. The perception probability to detect a meteor is very low at the egdes of your field of view compared to the very good perception near the center of the field. Therefore, if the clouds cover 20% near the edges you will not miss 20% of the meteors.
It seems hardly possible to consider the influence of the perception distribution over the field of view on the number of missed meteors during the observation. I can only recommend to rather underestimate the cloud coverage. A coverage of 50% means that you suppose 50% of the meteors to be missed (compared to clear skies). According to the perception distribution over the field of view this corresponds to a field of only 32° diameter (Koschack & Rendtel 1990a). This is as small as if you could only see the constellation of Cygnus. It will be very seldom that somebody continues to observe although he sees nothing but a part of Cygnus anymore. A cloud coverage of 50% corresponds to a correction factor of 2.0. Hence, I would not expect correction factors larger than 2.0.
Unfortunately, many observations were not usable in the past as they reported cloud factors of more than 2 and even more than 3 which means in the above example that the observer did not see more than a part of Cygnus throughout his entire observation.
Which items are of particular importance to record when plotting meteors?
The interest in plotting meteors and minor showers has grown lately. Therefore, I would like to give some details on meteor plotting. Naturally, the apparent path of the meteor is the most important information for the shower association or radiant investigations. Meteors can only be drawn on gnomonic charts, any other projection method causes the paths to be curved instead of straight lines. An ideal set of charts is the Atlas Brno 2000.0 created by Vladimir Znojil .
When you noticed a meteor you should remember the surrounding stars, which will, however, often be rather faint. The better you know the stars on your map, the closer are the stars you can use for plotting the meteor path. Otherwise, it will take you quite some time to identify the +5m stars on the map. Most observers do not have that extremely good knowledge of the sky; they will use more distant and brighter stars to plot the meteor. These stars should not be further away than some 15° . Otherwise, the accuracy would suffer from the same disadvantage like that of the counting method, that is prolonging the path over large distances on a sphere.
Since the direction of the path is essential for the association with a radiant, we should concentrate on this information when plotting a meteor rather than the length. It is helpful to remember two stars near the prolongation of a meteor, e.g. you could say in your mind, “a bit left of this star, but a bit more right of that star” with both stars being located at different distances from the meteor but near the prolongation. If you make such a relative positional estimate for four stars on both the forward and backward prolongation, the plot will have sufficient accuracy, although you did not use the +5m and +6m stars in the very vicinity of the meteor path. Alternatively, if the meteor appeared very close to a star which you easily find on the chart you can also use this star as a reference and define the direction by two further stars on the prolongation.
When you have drawn the meteor onto your map, you can note the characteristics of the meteor, starting with the time in hours and minutes and an estimate of the brightness of the meteor in half-magnitude steps. The Atlas Brno provides the observer with magnitude values at several stars on each of its charts. But do not note a magnitude of +2.2m when the meteor was as bright as gamma Cygni; just write “+2”.
The angular velocity should not be neglected when plotting meteors. It is a very helpful information when discriminating sporadic meteors from shower meteors. Although it seems difficult to estimate the speed in degrees per second I can only recommend this measure as described in another FAQ.
Finally, an estimate of the accuracy of the plot should be given. It is recommended to use a three-step scale from “1” (accurate plot) to “3” (uncertain path), or, alternatively, to note “+”, “o” and “-“. Generally, good marks will be given to meteors near the center of the field of view whereas bad marks will indicate meteors which were noticed at the border of the observed field.
When you got accustomed to plotting, and it takes you only half a minute or so to plot a meteor you should also give additional information like the color and persistent train when you were able to detect them. The following items are sorted by their importance for plotting meteors.
- apparent path, magnitude and (at least) time stamps about every 15 minutes
- angular velocity
- an estimate of the accuracy of the plot (e.g. 1, 2, 3)
- color, persistent train, other peculiarities
How do I get the Atlas Brno 2000.0?
You can download the atlas and print it yourself, but make sure you get the scale right! For reporting positional data it is important that the scale of the charts remains unchanged. This means the distance between the thin crosses has to be 70 mm. The origin of the X, Y coordinates is located in the bottom left corner of the chart (X-axis to the right, Y-axis upwards). The reference lines are the inner ones, not the thick lines. Photocopiers often tend to change the original scale. Therefore, use the originals you bought whenever you are going to make copies. Otherwise you merely add to the errors. If the scale error exceeds 3 mm over the whole length of the chart (i.e. if the distance between the thin reference lines differs by more than 3 mm from 280 mm for the short side and/or 350 mm for the long one) you should use another device.
- Znojil V., Gnomonicky Atlas Brno 2000.0, WGN 16:4, pp. 137-140.
- Arlt R., Frequently Asked Questions on Observing Methods, WGN 22:1, p. 6.
How do I measure meteor coordinates on the Atlas Brno? The origin is ambiguous.
Put the chart before yourself with the chart number in the upper right corner. The origin of the coordinate system is always in the lower left corner, exactly on the inner frame of the chart. The y-axis is directed upwards, the x-axis, to the right. Therefore, only positive numbers are valid. Charts 1,2,3 and 7,8,9,10,11,12 have landscape format, charts 4,5,6 have portrait format.
Coordinates are measured in millimetres. There is no need to give fractions of millimetres; the plotting accuracy is definitely worse than half a millimeter. Small crosses indicate distances of 70 mm. If you make copies of the charts for your own purposes note that most copy machines may change the scale of the charts, although you chose 1:1 on the panel. Generally, the size of the long axis changes by 1 to 3 mm. You can diminish the resulting error of the measurements if you use the small 70-mm crosses as auxiliary origins and add the known offset afterwards.
How reliable is the velocity estimate in degrees per second?
As shower association with both counting and plotting method depends on the meteor speed observers should try to estimate the velocity during the observation. In the past step scales from zero to five were used, now a direct estimation in degrees per second is recommended. It turned out that the estimates of medium experienced observers scatter by 30-50 per cent. The statistical uncertainty of the estimates is the same as that of step scales, however, the systematical error is smaller. (There is no definition on how wide are the steps of the speed scale and what is the offset from zero for the first step.) Remember that the maximum angular speed is about 40° /s due to geometrical reasons; most meteors do not exceed 25°/s. Note that the maximum speed observable of a shower with a geocentric velocity of about 35 km/s is 20°/s. Capricornids and kappa-Cygnids will hardly be faster than 10° /s. Such values restrict the range of speed estimates, reliability should be sufficient for shower association after some tens of meteor sightings.
How to treat meteors which could not be associated with a meteor shower unambiguously?
Observers regularly report about meteors which they saw just at the border of their field of view. It has been unclear how to associate these meteors with the radiants active at that time as the position of the path as it is stored in the observer’s mind will be very uncertain.
At first, we have to ask whether or not this meteor should be included in the reported number of meteors. The zenithal hourly rate (ZHR) is generally calculated for an unrestricted view. On the other hand, the computation of spatial number densities in meteor streams assume a diameter of the field of view of 105° (it is not 100° for statistical reasons). It turned out that the fraction of meteors seen outside this field is only about 2%. Hence, we may use the ZHR for further calculations requiring a restricted field of view. Note that a distance of 50° from the center of view is sometimes underestimated. If you watch Deneb (alpha Cyg) and see a meteor somewhere in Hercules, it is well within this radius. Your field extends to beyond the celestial pole, it includes the complete Pegasus and Aquila. Therefore, I recommend to record every meteor seen regardless of how far from the center of view it appeared.
(Sometimes you caught a glimpse of a light source in the very corner of your field of view, and you were not sure whether this was a meteor. Although it might be confirmed by fellow observers, you should not note the meteor as you would probably not have noted it when you were alone.)
When including far-distance meteors we have to deal with the problem of associating them with a shower radiant. The probability of a wrong association decreases with increasing activity of a meteor shower. Hence, during a major- shower watch you should count the meteor with unclear origin as a member of the major shower. When you are plotting meteors during a minor-shower observation you should not apply the direction criterion too strictly. A radiant diameter of 20° is appropriate for meteors which appeared 50° off- center of the field of view. Consider the angular velocity and the path length with higher priority for shower association as these criteria will be less severely affected by the distance than the orientation of your memorized path. (Note that the physical image in your eyes is heavily distorted at the edges, and the brain cannot reconstruct true angles correctly.)
Another frequent problem is the association of meteors with twin radiants such as the Taurids. Meteors may match the criteria of belonging to both showers, and observers ask what to do with these meteors. There are four possibilities to decide in this case:
- We put this meteor to the sporadics which is probably a bad solution,
- we put this meteor into a separate number of undiscriminated Taurids,
- we do decide in favour of one of the branches in any case or
- we put `half a meteor’ to either of the branches.
The solution (2) seems to be good as we apparently do not do anything wrong. However, the activity calculation of the separate branches is affected by this decision as not all meteors which belong to the Taurids will be incorporated. Hence we make a relatively large error when excluding them from both the northern and southern branch. Consequently, it seems better to include the ambiguous meteors in the separate branches in any way. A calculation of the total Taurid activity is possible with the solutions (2), (3) and (4). Now we have to choose between (3) and (4). Imagine a meteor which cannot be associated with one of the branches as the assumed errors in direction and speed do not seem to allow a discrimination. However, one of the radiants will have another distance to the meteor and, hence, another speed expectation or the meteor will hit one of the radiants slightly closer than the other in most of the cases. Consequently, there is a slightly higher probability for one of the branches to be associated with the questionable meteor in next to every case; we can associate the meteor with one specific branch. Although this decision is based on statistical assumptions, it is the method with the smallest error we can make.
As an example, we may consider a meteor in a distance of 40° from the radiant of the Northern Taurids, at an elevation of 90° . We assume an estimate of the angular velocity of 10° /s. Imagine that the path of of the meteor exactly aligns with both radiants of the Taurids. A calculation of the expected speed yields 12° /s for the northern and 14° /s for the southern branch. Although both radiants match the direction and speed criteria within the error limits, the northern branch fulfills the speed estimate closer than the southern one. Hence we do decide in favour of the Northern Taurids. Additionally, we should consider the path length. If the meteor had a short path, we should prefer the closer radiant.
If we put half a meteor into either branch’s number according to (4) we neglect the small differences in association probabilities. The method described in the previous paragraph can potentially uncover significant differences in the activity from both radiants when the meteor numbers are large enough. However, ‘bisecting’ meteors never produces a difference in activity, it even smears out differences produced by other meteors which could be associated unambiguously.
The following guidelines should be considered for meteor at the border of your field of view:
- Every meteor should be reported regardless of how far it appeared from the field of view.
- Do not report ‘meteors not analyzed’; decide unambiguously in favor of a radiant or the sporadics. Use the angular velocity and path length with high priority for ahower association. Remember that you would bias the activity calculation from shower numbers and sporadics.
- The designation ‘DIV’ is reserved for comprising several Aquarid radiants in July and August which could not be resolved due to a far field center.
- Do not report ‘STA’, ‘NTA’ and ‘TAU’ at the same time. In case you plotted the meteors try to discriminate both branches even if the probabilities of shower membership are only slightly different.
- Do not report half a meteor in the shower analysis.
Where do I send my observation results?
Preferably, use the Electronic Visual Meteor Observations Summary Form. You can also e-mail your reports to the Visual Commission Director.
If you don’t have internet access you’re probably not reading this, but you could still send in the paper version of the summary form.
How should the observing fields be distributed over the sky when observing in a group?
A group observation is much more fun than sitting alone somewhere in the field. Notwithstanding, there is no other difference between a group observation and that of a single observer. Each of the participants produces his own, independent observational record. There is no need to cover the entire sky with observing fields, neither is it recommended.
The observing fields are best placed when the observers can easily distinguish the meteor showers active. This would mean that all the participants of a group observation look into the same direction. This is in fact no problem as their average rates give a more singnificant value for the meteor activity than the rates of a single observer. On the other hand, if cameras are operated during the observation, it is very important to record the times of bright meteors which might be photographed. Observing in different directions increases the chance to get the appearance times for most of the bright meteors.
The following rules may be considered when planning a group observation:
- Experienced observers discriminating all the radiants active should face a direction close to the minor-shower radiants. During the Perseids this corresponds to fields in Pegasus and eastern Vulpecula which is close to the Aquarids and the kappa-Cygnids.
- Observers who only discriminate major-shower and other meteors can scatter their fields of view around the radiant of the major shower, in a distance of 20° to 60° . During the Perseids these fields can be, e.g., in Cepheus, Ursa Minor, Camelopardalis, Andromeda, Pegasus, and Pisces.
- No observer should face a direction which is 180° in azimuth from the radiant when the radiant is lower than about 50° . Do not use Aquila, Hercules, Ophiuchus or more southerly constellations during the activity period of the Perseids.
- No observer should look at fields with elevations lower than 40° .
- If cameras are operated or if the observation takes place in an area covered by network cameras, each observer should carry an accurately set watch to determine the appearance times of bright meteors with an accuracy of 1 s.
Frequently Asked Questions on Photographic Observations
Although I saw a lot of bright meteors during the night, I did not catch any on a photo. What did I do wrong?
The meteor limiting magnitude of photos is not the same as the limiting magnitude for stars. Meteors move about 2000 times faster (say 10° /s) over the sky than stars. Hence, each particle of the photographic emulsion is exposed to the light about 2000 times shorter. Consequently, the same blackness as stars is produced by meteors being 2000 times brigher. If the limiting magnitude for stars is +6m on the photo you may not expect meteors on fainter than -2m. If the meteors are very slow and have long-persisting trains, you may get meteors up to 0m at best. Furthermore, the photographed region of the sky with normal lenses (f=50 mm) is much smaller than this of a visual observer. Even if you saw two or three fireballs during the night they are still likely to miss the camera field on the sky.
Good results can be obtained with fish-eye lenses which cover the whole sky. You will be successful in a major shower’s maximum night with this equipment. The advantage of photographing the complete sky combines with reasonable limiting magnitudes and sufficient accuracy. alpha-Capricornids, kappa-Cygnids and chi-Orionids are known for bright meteors. They move very slow making it more probable to be photographed.
Frequently Asked Questions on Fireballs
- How bright does a fireball have to be to be classed as a fireball?
- How can I estimate the brightness of a fireball?
How bright does a fireball have to be to be classed as a fireball?
There is no natural difference between fireballs and ordinary meteors. Therefore, it is a pure definition when we say: A fireball is a meteor with magnitude -4m or brighter. If you observe such a bright meteor please report this to the Fireball Data Centre (FIDAC) of IMO which stores each and every fireball in a database. The next paragraph tries to help with estimating a fireball’s magnitude. As the estimates will be rather uncertain FIDAC also stores meteors which were estimated to be -3m. If you see such a “semi-fireball” you should also contribute to the database with this meteor.
How can I estimate the brightness of a fireball?
Meteors with brightnesses up to 0m can be estimated with sufficient accuracy. When it comes to events with negative magnitudes the observer will not have enough reference objects. Sirius (alpha CMa) has some -1.5m but is a good reference star for observers on the southern hemisphere only. Then there are the planets which have the stain that their brightness changes depending on the position to the Earth. Jupiter has -2.2m at best, generally has around -2m. Venus can be well brighter than -4m and reaches its maximum brightness near the utmost elongations when it is most likely to serve as a reference object.
Brighter objects will hardly be present when observing as moonlight generally causes you not to observe. Hence, you have to extrapolate the brightness from visible objects or from well-known impressions like that of Venus though it may not be visible. The scale of astronomical magnitudes is logarithmic. Note that a fireball with twice the brightness of a visible object exceeds it by less than one magnitude. An object being 4 magnitudes brighter than a reference object produces 40 times more light.
As bright fireballs usually illuminate the surroundings, try to remember whether the meteor caused shadows. If so the fireball had at least magnitude -5. If you see a fireball directly in the centre of the retina consider the effect of blinding at this point. When the fireball vanishes try to perceive stars in the very centre of the retina. If you cannot even see +2m stars the meteor had surely -10m.