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On Sep. 10, 2018 comet 21P/Giacobini-Zinner (P=6.54a) will pass perihelion (in the distance of 1.01 AU) for another time. On the same day it will reach perigee at a distance of only 0.39 AU. In those days it is predicted to reach magnitude 7.0. According to my empiric formulae a coma diameter of about 12' and a tail of length 0.5° can be expected. The comet should be brighter than 12 mag between mid-June and mid-December. During this interval it will move through the constellations Cygnus, Cepheus, Cassiopeia, Camelopardalis, Auriga (perihelion/perigee), Gemini, Monoceros, Canis Major and Puppis. From mid-European locations the comet can be followed in the morning sky until the opening of November. Earth will cross the orbital plane of the comet on Oct. 9.
The comet showed a different activity pre- and post-perihelion, with activity parameters above average during the whole apparition. Surprisingly, post-perihelion the activity parameter was larger than pre-perihelion. Probably a region of enhanced activity slipped into Sun's shadow following perihelion. The maximum brightness of 7.2 mag was reached around Sep. 10, 2018. Based on 628 observations from 67 observers the development can be well described by the following formulae:pre-perihelion: m = 9.0 mag + 5×log D + 17×log r post-perihelion: m = 9.4 mag + 5×log D + 21.5×log r
Between mid-May and the end of July 2018 the apparent coma diameter increased slowly from 0.5' to 6', then growing faster, reaching 10' in mid-August. The drop at the end of August may have been caused by the influence of the disturbing Moon. Actually the maximum may have last from mid-August to mid-September. Thereafter the apparent diameter decreased to 6' by the end of September, 3' by mid-October and 0.5' at the start of February 2019. Between mid-May and mid-August 2018 the absolute coma diameter increased very steadily from 40.000 km to 210.000 km. Thereafter it started to decrease, reaching 170.000 km by mid-September, 80.000 km in mid-October and 30.000 km at the start of February 2019.Total Brightness and Coma Diameter
During its approach to the Sun the coma got more condensed, with the degree of condensation raising from DC 3 in mid-June to the maximum value of DC 5, which was hold constant between early August and mid-September. Thereafter the coma got more diffuse, with the degree of condensation estimated at DC 3 in mid-October. A tail was reported between early July and mid-October, but it reached significant length only between early August and the end of September. Between mid-August and mid-September the estimates reached up to 40' (900.000 km). Its orientation changed from SE to SSW during its visibility.