On Aug. 10, 1998 the Australian amateur Peter Williams accidently discovered a 9.5m comet near the borders of Triangulum Australis/Circines in the course of his variable star programme. Comet C/1998 P1 (Williams) showed a diffuse 4' coma with a 13m central condensation (IAUC 6986/AFZ 610). Additional observations indicated a fast motion northward and a perihelion in October. It was proposed that this comet would be accessable for Europeans around the end of November.
The person who recovered this comet first after perihelion was Alan Hale on Nov. 15. On Nov. 22 he was able to estimate the comet with 10x70 binoculars as 9.4m (IAUC 7058), nearly a magnitude fainter than expected. The first estimates of observers of the Fachgruppe Kometen dropped in in mid-December, indicating a magnitude only 0.5m below the expectations. This was a first hint that the brightness would decrease slower than for an average comet after perihelion, which can be confirmed by 44 observations of 10 FG observers and 150 international estimates.
Contrary to the post-perhelion scenario pre-perihelion the brightness did show a fast increase, as the small number of observations indicates. Post-perihelion the absolute magnitude was fainter and the activity parameter n smaller. Consequently the first observations after the conjunction with the sun did show a fainter object than expected. But during the following weeks it approached the predictions and thereafter was even brighter than expected. Since the beginning of February however, the distance sun-comet as well as earth-comet increases, resulting in a much faster decline of the brightness. The derived formulae are:
The diameter of the coma was relatively constant at 5' before perihelion, post-perihelion it rose from 3.5' to 5' at mid-February. Shortly thereafter it decreased to 2', reaching 1.5' at the end of visibility. The absolute coma diameter rose steeply from 200.000 km to 330.000 km before perihelion. After perihelion it shrunk gradually from 310.000 km to 200.000 km. Pre-perihelion the coma was well defined (DC 5), after that it was more diffuse with the degree of condensation decreasing from DC 3-4 to DC 1-2. Observations of a tail were very rare with a length not exceeding 0.25° (about 2 Mill. km).
Total Brightness and Coma diameter
Andreas Kammerer
