Analysis of Comet Apparitions

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On Dec. 12, 2018 comet 46P/Wirtanen (P=5.44a) will pass perihelion (solar distance: 1.06 AU). On Dec. 16 it will reach perigee at a distance of only 0.078 AU. In those days it should peak at about magnitude 4.0, thereby moving with up to 10'/h in front of the stellar background. According to my empiric formulae a coma diameter of about 55' and a tail of length 1° can be expected in this case. After perigee the comet is expected to fade rapidly. It should be brighter than 12 mag between September 2018 and March 2019. During this interval it will move through the constellations Cetus (southern part), Fornax, Eridanus, Taurus (perihelion/perigee), Auriga, Lynx, Ursa Major and Leo Minor. From mid-European locations the comet can be glimpsed low above the southern horizon around midnight between September and December. Thereafter it rapidly gains altitude, reaching the zenith around Christmas. In March 2019 it will be best observable before midnight. Earth will cross the orbital plane of the comet on Dec. 15.

Comet Wirtanen fulfilled the expectations and was observable without optical aid throughout December, when it stood as a moon-sized "cloud" in the sky, changing its position on a daily basis due to the close proximity to Earth. As expected the brightness decreased rapidly following perihelion and perigee. In addition, the coma got more diffuse and the altitude above the horizon decreased, so that the comet wasn't easy to find even with binoculars by mid-January 2019. Despite these unfavorable conditions the comet was observed until the opening of April, then of magnitude 12.5. Based on 545 observations from 56 observers, a different brightness development pre- and post-perihelion results as is well shown by the two formulae:

pre-perihelion: m = 9.4 mag + 5×log D + 21.5×log r
post-perihelion: m = 9.6 mag + 5×log D + 11.2×log r

Based on these formulae a maximum brightness of 4.3 mag on Dec. 16, 2018 results. However, the estimates even indicate a maximum brightness of 4.0 mag.

Total Brightness and Coma Diameter

At first the apparent coma diameter increased continuously from 0.6' in mid-September 2018 to almost 40' at the end of November, only to increase even more rapidly to its maximum value of 75' shortly before perigee. Thereafter it decreased rapidly, reaching 45' on the Christmas days, followed by a slower decrease to 35' at the start of January 2019, 15' at the beginning of February und only 2' at the start of April. The absolute coma diameter increased continuously at first from 45.000 km in mid-September 2018 to 275.000 km in mid-November, remaining at this value quite constant until around Dec. 10, followed by an apparent decrease to 200.000 km until the Christmas days. Thereafter the coma seems to have increased again, reaching about 260.000 km in mid-January 2019. During the following weeks it eventually decreased, measuring 200.000 km in mid-February and only 100.000 km at the opening of April.

Throughout the apparition the coma was only moderately condensed, but showed a prominent central condensation. The degree of condensation remained at DC 2-3 until the end of October 2018, increasing to DC 4 until the days around perihelion. Immediately thereafter the coma started to get more diffuse, reaching DC 1 at the start of April 2019. The narrow gas tail was not recognized visually.

Andreas Kammerer

FGK observations