Piscids

Observer's Synopsis

This weak and diffuse annual shower is active during the period of August 12 to October 7. The peak intensity of about 5 per hour extends from about September 11 (Solar Longitude=168 deg) until about September 20 (Solar Longitude=177 deg). The average radiant is RA=0 deg, DECL=+4 deg. A probable northern branch might reach maximum around October 12 (Solar Longitude=198 deg) from RA=26 deg, DECL=+10 deg.

History

Usually considered the "highlight" of September, this meteor shower, like all showers visible during the month, has been poorly observed due to an overall lack of observations. It seems apparent that the radiant is quite diffuse, though additional observations might help to resolve the stream's possibly complex structure.
Only two apparent observations of this shower seem to exist in the 19th Century and they were both made by William F. Denning (Bristol, England). The first observation came on September 14-15, 1879, when he plotted seven "very slow" meteors from RA=1 deg, DECL=-5 deg. The next observation involved the plotting of five meteors from RA=4 deg, DECL=-2 deg during September 3-5, 1885. Although both radiants were called "Piscids," neither Denning or any other astronomer recognized this as a possible annual shower. The Author notes that both radiants barely qualify as representing this stream.
Cuno Hoffmeister was the first to officially recognize the Piscid stream. Analyzing the observations made by German observers between 1908 and 1938, he identified numerous probable observations of this shower. He concluded that the duration extended from August 16 to October 8, while maximum came on September 12 (Solar Longitude=169 deg) from RA=0 deg, DECL=+4 deg. Hoffmeister described the shower as distinct, but weak. He added that two radiants were possibly present---both moving parallel to one another, but one located north of the ecliptic and the other south.
Although Hoffmeister gave no exact details about his possible northern and southern Piscid showers, a table was given which demonstrated the general motion of the radiant. The Author notes some interesting patterns in this table. First, between August 16 and September 10, the declination changed only slightly, with the average being about +1.2 deg (based on 49 radiants). On the other hand, the declination for the period September 16 to October 8 averaged +8.1 deg (based on 38 radiants). The number of observed radiants remained fairly consistent during September 1, 8, 12, and 18, which may indicate a fairly flat maximum, rather than a distinct peak. On September 29, the number of observed radiants reached its highest point. Blending this declination and activity analysis, the Author believes Hoffmeister's data points to a southern stream reaching a flat maximum during the first half of September, while a northern branch is active around September 29. Whether the September 29 peak was due to a maximum of the northern branch or simply a combination of both branches cannot be determined.
The addition of photography in the field of meteor studies had began during the 1890's, but the Harvard Meteor Project of 1952-1954 was the most ambitious program. From this survey came the orbits of over 2500 meteors, which supplied a very large database for future studies of meteor showers. During 1971, Bertil-Anders Lindblad began a computerized study of these meteor orbits in an attempt to isolate active streams.
Lindblad detected two streams which he called the "Piscids"---each receiving a provisional stream number of "31" and "92." Stream 92 was given a long duration extending from August 31 to November 2. The radiant was given as RA=10 deg, DECL=+6 deg. But while this stream possessed similar characteristics to Hoffmeister's Piscid stream, the orbit revealed several strong dissimilarities, notably a 20 deg difference in the argument of perihelion, a 21 deg difference in the ascending node, and a 0.12 AU difference in the perihelion distance. Stream 31 seemed a more likely candidate, although it did occur one month later than Hoffmeister's stream and possessed a fairly short duration of September 25-October 19. The indicated date of the nodal passage was October 13 (Solar Longitude=199.1 deg), at which time the radiant was at RA=26 deg, DECL=+14 deg.
During 1973, Lindblad, as well as Allan F. Cook, Brian G. Marsden, Richard E. McCrosky and Annette Posen reevaluated several of the streams isolated by Lindblad's 1971 study. One of the most important discoveries was that over half of the meteors representing stream 92 were actually the Andromedids (see November). The authors concluded that the remaining meteors of stream 92 and the meteors of stream 31 formed the respective southern and northern branches of the Piscids. Although this seemed to confirm Hoffmeister's 1948 suggestion of two Piscid streams, the Author still has doubts about the validity of stream 92.
The 1971 study listed 33 members of stream 92, but this was whittled down to 14 meteors in 1973 after the Andromedids were subtracted. The Author believes 4 of the remaining "Southern Piscids" are actually members of the Southern Taurid stream of November. The remaining 10 meteors were considered a family by the authors of the 1973 study simply because their longitudes of perihelion were very nearly the same. In actuality these meteors change in perihelion distance from 0.25 to 0.66 AU and in argument of perihelion by 50 deg. From a study of the Perseids, Delta Aquarids, Orionids, and Geminids, the Author believes meteors should be assigned to particular streams if all orbital elements, except that of the ascending node, are a close match---especially when dealing with showers of long-duration. Applying this theory, the Author notes that 8 of the 10 meteors assigned to the "Southern Piscids" form two separate streams (one of which is the October Cetids)---neither of which seem to represent the observations of the Piscids noted by Hoffmeister. In fact, no convincing photographic Piscid appears in the 1952-1954 Harvard Meteor Project, partially due to the cameras being shut down during September 2-9.
The Piscids were well represented during the two sessions of the Radio Meteor Project conducted by Zdenek Sekanina during the 1960's. During the 1961-1965 survey Piscid meteors were detected during August 14-October 4. The date of the nodal passage was calculated as September 10.7 (Solar Longitude=167.6 deg), at which time the radiant was at RA=359.8 deg, DECL=+3.4 deg. The link with Hoffmeister's Piscids was considered very good (D-criterion of 0.110). The 1968-1969 session revealed Piscids over the period of August 12 to October 7. The nodal passage was said to have occurred on September 16.1 (Solar Longitude=172.8 deg), at which time the radiant was at RA=8.5 deg, DECL=6.9 deg.
Sekanina's radio-echo data seem to indicate that a long-duration shower does reach maximum in the Pisces region in early September, however, even though there was no trace of a secondary stream, the orbits obtained during the 1961-1965 study and that of 1968-1969 were somewhat different. The Author believes these orbital differences, as well as the differences in the dates of nodal passage, offer supporting evidence that two branches of the Piscid stream do exist, but that their production of very diffuse and weak showers make them difficult to separate when comparing orbital elements. It is probable that only a close monitoring of activity levels will properly separate these two streams.
Recent observations of this shower have been made by members of the OrganizationsWestern Australia Meteor Section. According to section director Jeff Wood, the September full moon of 1979 was followed by observed activity from the Delta Piscium region during September 22-23. A maximum ZHR of 1.32+/-0.22 came on September 23 from an average radiant of RA=10 deg, DECL=+11 deg. During 1980, Piscids were detected during September 10-12. A maximum ZHR of 1.24+/-0.26 was observed on September 11 from RA=4 deg, DECL=+9 deg. In addition, the 1980 observations also revealed showers which were referred to as the "Southern Piscids" and "Northern Piscids." The former shower was observed during September 11-27. A maximum ZHR of 2.08+/-0.20 came on September 19 from an average radiant of RA=4 deg, DECL=0 deg. The "Northern Piscids" were detected during October 5-16. A maximum ZHR of 2.94+/-0.25 came on October 12 from RA=26 deg, DECL=+8 deg. Although the latter shower seems to definitely represent an observation of the northern branch of the Piscid stream, the Author believes the "Southern Piscids" might not actually be associated with the southern branch.

Orbit

The orbit of this stream was first determined by Hoffmeister. Based on visual radiants, it is

AOP AN i q e a
296 169 3.5 0.40 0.72 1.43

What later became known as the "Northern Piscids," stream number 31 of Lindblad's 1971 photographic meteor survey possessed the following orbit.

AOP AN i q e a
290.8 199.1 3.4 0.399 0.797 2.062

Two orbits were determined by Sekanina from data gathered during the two sessions of the Radio Meteor Project. The first orbit represents the material obtained during the 1961-1965 survey, while the second orbit was computed using 1968-1969 data.

AOP AN i q e a
298.5 167.6 3.8 0.344 0.816 1.868
306.3 172.8 3.5 0.311 0.769 1.349

The Author has collected seven photographic meteor orbits from MP1961 and B1963. The average stream orbit follows:

AOP AN i q e a
306.6 174.7 6.5 0.271 0.853 1.844

If two separate branches are present within the Piscid stream, their presence might best be demonstrated by comparing the values of the Argument of Perihelion (AOP) and the perihelion distance (q) above. Admittedly, these orbits do not confirm Hoffmeister's belief that one radiant lay north and the other south of the ecliptic.

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