Path and radiant

As you know from major-shower observations, shower meteors seem to radiate from a certain point in the sky, the radiant. To gain some insight into this, please turn to Figures 8 and 9 which show how observers A, B, C, D and E project a meteor M onto the sky hemisphere as sections of arcs. These great circles concern the meteors belonging to the same radiant and have two poles or intersection points in common: the radiant R above the horizon and the anti-radiant AR below. The plane perpendicular to the great circles through O marks the points at 90° distance from the radiant.

When the chart's center P is situated at less than 90° from the radiant, the latter may be marked on the same plane as that of the map by use of the usual projection formulae. The backward extensions of the shower meteors meet the radiant. For your convenience the Shower Calendar includes a table giving the positions and drifts of the radiants.

When the radiant is at more than 90° from the map's center, it cannot be projected onto this chart, but the anti-radiant may be. Its position results from:

delta_AR = - delta_R
alpha_AR = alpha_R ± 180°

Now the forward prolongation of the meteor trail meets the anti-radiant. This also become obvious from Fig. 9.

Figure 8 - Side view of the trajectory being projected on the celestial sphere and on the map. P denotes the map center (= projection center), and M is the meteor trajectory.

Figure 9 - View of the sphere on which the meteor's trajectory was projected from different places. A, B, and C are situated on the same meridian (great circle). Other observers (D, E) may see it on other great circles.

Due to perturbations and different ejection conditions from their parent object, the individual particles of a meteor shower do not move on exactly the same orbit. Thus they do not all enter the atmosphere exactly parallel to one another. The result of this is that a radiant is not a point but an area of a certain size whose dimensions depend on how widely the individual orbits are spread, and on the geometrical conditions of how the stream encounters the Earth, and thus differs from shower to shower (Kresák & Porubcan, 1970).

If you plot shower meteors, their backward extensions will form a radiant area considerably larger than the radius obtained from photographic observations. This is due to plotting errors. These errors will be reduced for experienced observers but generally they cannot be avoided. When trying to decide whether a meteor belongs to a certain shower or not the question arises as to how large a radiant area can safely be assumed. Making it too large means the sporadic pollution becomes too strong, i.e. several sporadic meteors will meet the radiant accidentally. If the radiant area is too small a certain number of shower meteors will be classified as sporadics because of the plotting errors. The optimum radiant diameters for shower association are given in Table 6 and 7.