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Comet 71P/Clark (P=5.56a) will pass perihelion for another time on June 30, 2017, expected to reach 10.5 mag around this date. However, it will be positioned in the southern part of the constellations Scorpius and Sagittarius, thus not being reasonably observable from mid-European locations during the most interesting weeks. Only in November will it have gained enough altitude for meaningful observations. However, at this time it should have faded to magnitude 13.

The comet performed slightly more positive than expected. The brightness development was different pre- and post-perihelion, as is shown by 124 observations from 27 observers. It can be well represented by the following time-dependent formulae (standard formulae do not work nearly as well):

pre-perihelion: m = 11.1 mag + 5×log D + 0.031×|t-T|
post-perihelion: m = 11.1 mag + 5×log D + 0.010×|t-T|

The comet peaked at 10.0 mag during the first week of July. At the end of October it had faded to 13.0 mag.

Total Brightness and Coma Diameter

The apparent coma diameter increased from 0.6' at the beginning of the visual apparition to the maximum of 2.5' in mid-July. Thereafter it decreased much slower, reaching 1.1' at the end of October. The absolute coma diameter increased from 25.000 km at the start of the apparition to the maximum of 100.000 km. This value was attained in mid-August and hold at least until the end of October. The coma was most condensed at the start of the visual apparition (DC 5). This value of the degree of condensation was nearly constant until mid-July. Thereafter it decreased slowly, reaching DC 2 at the end of October. No tail was discerned visually.

Andreas Kammerer