Comet 78P/Gehrels was observable with average sized instruments in the evening sky of fall/winter 2004/05. 48 observations by 7 members of the German Comet Section and 220 international reports indicate that the brightness evolution can be best described by time-dependant formulae, with a transition interval of about two weeks around the date of perihelion::
Total Brightness and Coma Diameter
The maximum brightness of 10.1 mag was reached in mid-November 2004. Taking into account the brightness increase shortly after perihelion the maximum brightness of 10.0 mag was reached in mid-November 2004. The apparent coma diameter increased from 0.7' at the start of the apparition to a maximum of slightly more than 2.5' during the second half of November (which was hold for two months). Since the first half of January 2005 it shrinks, reaching 0.6' at the end of March. The absolute coma diameter increased from 60.000 km at the end of July 2004 to 150.000 km in mid-January 2005. Thereafter it began to decrease slowly, measuring 75.000 km at the end of March. The degree of condensation increased from DC 2-3 at the end of Juli 2004 to a maximum value of DC 5-6 in mid-November. Thereafter the coma grew more diffuse, reaching DC 2 in the first days of March 2005. For a long time the coma showed a prominent small central condensation with a false nucleus and a very diffuse outer coma. Visually a tail could be seen - at times rather easily - between September 2004 and mid-January 2005, but with a length never exceeding 5' (almost 1 Mio. km).
Andreas Kammerer
