S

0.25 ±0.01

0.26 ±0.01

-0.16 ±0.01

0.38 ±0.01

-0.43 ±0.01

Daily motion of shower radiant parameters. Days are given in the approximately equivalent degrees of mean solar longitude rendering the motion measurements unit-less.

Table 2

Daily motion of shower radiant parameters. Days are given in the approximately equivalent degrees of mean solar longitude rendering the motion measurements unit-less.

The radiant motion measurements are dependent on the meteors selected on which to perform these measurements; the selection of these shower meteors is made using some assumed daily motion in the radiant position. This intertwined relationship allows the measurements to easily vary even on the same data set. The most important feature of the measurements are their use in correcting the perceived motions in the current data set. However, in order to verify that these are close to those expected from other surveys, bearing in mind the measurement instabilities mentioned above, comparisons are now made. Cook [3] lists a daily motion for the sda of 0.8° d_1 in right ascension and 0.18° d_1 in declination which is similar to that obtained here. More recently Kronk [5] and Rendtel, Arlt and McBeath [9] list this motion at 0.9° d_1 in right ascension and 0.4° d_1 in declination. As the amor values are based on a larger sample of orbits than these, with correspondingly small uncertainties, it is more likely that the amor low uncertainty values are closer to the "true" motion. The eta right ascension measured daily motion is close to Lindblad's [7] 0.76° d_1: however, that in declination diifers to a greater extent from Lindblad's 0.422° d_1 motion. Lindblad only covers solar longitudes from 43° to 47° with 23 single data points, while the current study uses ~ 103 points over a longer period albeit with higher individual uncertainties. The ~ 0.1 longitudinal motion found for the cap is approximately that expected according to Cook [3]. There appear to be no published daily motion for comparison with the dsx, this shower shows less longitudinal motion with respect to the Sun than do the eta and sda.

In addition to the radiant position other orbital parameters also experience measurable daily changes. Sometimes these are very small but in a number of cases they are clearly important. The sda provide a good example of the latter; the perihelion distance (q) and argument of perihelion (w) orbital elements of its members exhibit clear daily motion as shown in Figure 6. The fitted lines in the latter do not appear to be correctly centred in some cases, this is due to the increased uncertainties at higher q and lower w respectively biasing the fitting routine (the median Aq is 0.014 AU for qN < 0.07 AU and 0.019 AU for qN > 0.07 AU; the median Aw is 3.2° for ujn < 155° and 2.2° for coN > 155° (qN and ujn are motion corrected values)). The reason for such daily motions can be clearly illustrated; for example, Figure 7 shows that as the component of the meteoroid's pre-impact heliocentric velocity in the direction of the apex of the Earth's way (Vx) increases the range of shower meteoroid q and u allowed changes in a very constrained fashion. As the Earth moves in its orbit the direction of the apex does change, hence the average Vx measured from a coherent stream source also changes leading to the daily motion in the orbital elements shown.

For any parameters (P) showing a clear daily motion, as measured by linear least-squares (dP/d\Q), all shower meteors are reduced to a central time (Aq) using

dXp)

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