Shower Analysis

In Section 6 you have learned about the criteria a meteor has to meet before it can be considered as a highly probable shower member. Now the practical use of the criteria will be examined. On a free afternoon you should allow enough time to analyse your observation.

First of all, take the IMO's working list of meteor showers and select all the showers active on the date of your observation. Then plot their radiant positions valid for that date onto the charts. Take into account the radiant drift by using the drift tables!

angular velocity [°/s]   5    10    15    20    30
permitted error  [°/s]   3     5     6     7     8

Table 3 - Error limits for the angular velocity omega to be assumed for shower association.

radiant distance     optimum radiant
of the meteor [°]    diameter [°]
    15                 14
    30                 17
    50                 20
    70                 23

Table 4 - Optimum radiant diameters to be assumed for shower association of minor shower meteors to well- and moderately defined radiants as a function of the radiant distance of the meteor.

radiant distance of the meteor   15°       30°       50°       70°

delta-Cancrids (DCA)           20°/13°   24°/18°   26°/21°   34°/30°
Virginids (VIR)                30°/20°   31°/23°   33°/26°   40°/34°
Sagittarids (SAG)              30°/20°   31°/23°   33°/26°   40°/34°
Southern Taurids (STA)         20°/13°   24°/18°   26°/21°   34°/30°
Northern Taurids (NTA)         20°/13°   24°/18°   26°/21°   34°/30°
Puppids (PUP)                    20°       24°       26        34°

Table 5 - Optimum radiant sizes (in alpha, delta) to be assumed for shower association of complex radiants.

The radiant diameters obtained in the previous section are plotted as concentric circles (or ellipses for the complex radiants) onto the chart (see Tables 4 and 5). The scale is not constant over the chart. Therefore, a radiant of 20° diameter in the center of the chart appears smaller than one of the same diameter near the edge of the chart. Table 6 shows the variation of scale over the chart. As an example, a radiant of 20° diameter at 150 mm distance from the center of the chart appears as a circle of 104 mm diameter.

d [mm]          0     50     100     120     150     170     200     220

s [mm/°]       2.8   3.1     3.9     4.4     5.2     5.9     7.1     8.1

Table 6 - Scale s in mm/° for different distances d to the center of the Gnomonic Atlas Brno charts (valid for the original projection radius R = 160.43 mm).

After plotting the radiants onto the charts you have to analyze each meteor separately. Use a ruler of at least 30 cm length to extend the meteor path backwards. If the backward extension meets a radiant area of the corresponding size the meteor may belong to this shower, i.e. the necessary condition that the "path can be extended backwards to the radiant" is fulfilled. It may happen that a meteor belongs to a radiant that is plotted on a neighbouring chart. The backward extension is prolonged onto this other chart by using stars which are present on both charts, i.e. stars in the overlapping region. You have to identify stars crossed or closely passed by the line on the neighboring chart and plot the line in the corresponding position to these stars onto the second chart.

In the next step, we analyze the condition that "the path length must be shorter than half the angular distance of the radiant to the start point". Since this is a rough measure only, we can neglect the variable scale of the chart. The distances are measured using the ruler and compared. If the condition is fulfilled we can check the next condition.

It reads "the angular velocity has to correspond to the expected value within the error limits given in Table 3. To check this condition you need a planisphere valid for the latitude of your observing site to determine the elevation of the start point of the meteor, and Table 2. Besides the start elevation you also have to determine the angular distance between the radiant and the end point of the meteor.

You may estimate the radiant distance on the planisphere with sufficient accuracy. Alternatively, estimate the distance of the meteor to the radiant comparing with distances between stars as given in the following table.

Chart 1  alpha Per  -- alpha Aur   20   Chart 5  alpha Boo  -- alpha CrB   20
         alpha Per  -- alpha UMi   40            alpha Boo  -- alpha Vir   35
         alpha Per  -- alpha Cyg   60            alpha Vir  -- eta UMa     60

Chart 2  alpha UMa  -- eta UMa     25   Chart 6  eps Peg    -- alpha Aql   30
         alpha UMi  -- alpha Aur   45            alpha Aql  -- alpha Cyg   40
         alpha Gem  -- beta Leo    60            alpha And  -- alpha Aql   65
         alpha UMi  -- beta Leo    75
                                        Chart 7  alpha Tau  -- Pleiades    15
Chart 3  alpha UMi  -- beta UMi    15            alpha And  -- beta Cet    50
         alpha Cyg  -- alpha Lyr   25            alpha And  -- alpha Tau   60
         alpha Lyr  -- alpha CrB   40
         alpha Cyg  -- eta UMa     65   Chart 8  alpha Leo  -- beta Leo    20
                                                 alpha Leo  -- alpha CMi   35
Chart 4  alpha Gem  -- beta Gem     5            beta Crv   -- beta Gem    85
         alpha Ori  -- beta Ori    20
         alpha Tau  -- alpha CMi   45   Chart 9  alpha CrB  -- alpha Her   25
         alpha Aur  -- alpha Ori   55            alpha Aql  -- alpha Her   40
                                                 alpha Aql  -- alpha Sco   60

Table 7 - Distances between bright stars as shown on Atlas Brno charts. The distances are rounded to 5-degree steps.

Next, determine the elevation of the start point using the planisphere. Now take Table 2 and determine the expected angular velocity according to the entry velocity V, radiant distance D and elevation h_b which we just obtained. This value has to be compared to the one you estimated during the observation. If the difference lies within the error limit, the condition "angular velocity" is fulfilled. The error limits to be used are shown in Table 3. The table has to be read as in this example: "If the angular velocity was expected to be 15°/s, the condition is fulfilled if the estimated value lies in the range 9-21°/s."

If the meteor fulfills all three criteria it can be considered a shower member. Sometimes it may happen that a meteor fulfills the conditions for shower membership of two different showers. In this case you have to choose the most probable shower, i.e. that shower the conditions are best fulfilled for. It is impossible to consider one meteor to be a member of two or more different showers. You must decide in favour of one shower even if the probability of the meteor's belonging to two showers is similar.