At the end of 2003 comet 2P/Encke made its 59th observed perihelion passage since its discovery in 1786. This apparition favored observers in the northern hemisphere. Comet Encke became observable (in Aries/Triangulum) in September 2003. In mid-November this comet passed earth at a distance of only 0.25 AU. At this time it moved from Cygnus towards Ophiuchus, thereby nearly approaching the zenith. Thereafter it plunged towards the western evening horizon and disappeared during the first week of December. According to my empiric formula it should have reached a maximum coma diameter of about 15' during the second half of November.
For the analysis 24 visual observations by 6 members of the german comet section plus 145 reports by international observers were taken into account. The first visual observations were obtained on Oct. 15 when the comet was only of magnitude 13.5. During the following seven weeks it rapidly increased in brightness and was lost as a 6.0m bright object in the evening twilight. The mathematical representation of the brightness evolution is complex. Assuming a log r-relation, two different formulae are necessary:
t < -57d: m = 8.4m + 5×log D + 37× log r
t > -57d: m = 10.6m + 5×log D + 13.6× log r
These formulae simulate the brightness evolution of the comet pretty good until its disappearance; however the deducible brightness of 4.0m at perihelion is too high. An alternate formula is:
m = 3.2m + 5×log D + 7.5× r
This formula yields a more plausible perihelion brightness of 5.5m, but deviates from the observed values by a greater degree at the beginning of the appearance (which is not significant since these estimates have a greater uncertainty anyway) and around the date of the above mentioned break in the brightness parameters.
Total Brightness and Coma diameter
The coma diameter increased equally rapidly until Nov. 25, starting at 1' (25.000 km) at the beginning of the appearance to a maximum value of 18' (215.000 km) one week after closest approach to Earth. Thereafter it decreased until its disappearance to 7' (125.000 km). The coma was very diffuse at most days. The degree of condensation remained almost unchanged at DC 2 until the end of November, thereafter increasing to DC 4. A small central condensation, at times clearly off-center, was observed most of the time.
Andreas Kammerer
