THE ACTIVITY AND ORBIT OF THE PERSEID METEOR STREAM

The activity profile and orbit of the Perseid meteor stream is investigated based on 605 photographic orbits obtained at various meteor stations in the per-iod 1937-1985. The study shows that the derived activity profile of the stream depends strongly on the resolution in solar longitude used by the investigator. If the activity profile is investigated at a high resolution in solar longitude two distinct Perseid maxima appear, one at solar longitude 138.97-degrees and a second at 139.61-degrees A more detailed investigation shows that the Perseid maximum has gradually changed with time. being located at approximately 139.5 139.6-degrees in the period 1940 1969 and at 138.9-139.0-degrees in the period 1970-1989. We interpret this shift as indicating a gradual increase in the ratio of new to old cometary material as comet 1992t (Swift-Tuttle) approached perihelion. An investigation of the scatter in the orbital parameters of the two maxima shows that the scatter in all orbital elements is larger in the ''old'' peak. The mean orbit of the stream based on all 605 orbits agrees well with that of comet P/Swift-Tuttle (1992t), The main discrepancy is in the ascending node and in the argument of perihelion, where the comet differs by about 0.40-degrees and 2.4-degrees, respectively. from that of the meteor stream. However, if separate mean orbits are computed for the two Perseid maxima. an orbit determination based only on the ''new'' maximum reduces these differences to 0.21-degrees and 1.33-degrees, respectively. The present study concentrates on bright meteors obtained in photographic two-station studies. It is likely that a similar two-peak structure can be detected in visual and radar data as well.