2.1. Astrometry One of the sources of the erratic behavior shown in Fig. 1 is the poor quality of the astrometry used to compute the orbital solutions. Astrometry of active comets is often affected by an asymmetric distribution of light from the coma surrounding the nucleus. If a model designed to fit the symmetric light distribution from a point source is used on an active comet, the fitted photocenter can be dependent on the size of the synthetic aperture utilized. This effect was quite obvious in the astrometry of comet P/Wild 2, ob- tained in support of the Stardust mission’s flyby of that comet (Tholen and Chesley, 2004). As the size of the synthetic aperture increased, the measured photocenter moved in the tailward direction. At least in the case of P/Wild 2 and over the range of synthetic apertures tested, the offset showed a fairly linear dependence on synthetic aperture size, so the position of the cometary nucleus was computed by linearly extrapolating the fitted coordinates to a theoretical zero-sized synthetic aperture. The resulting astrometry enabled the uncertainty in the position of P/Wild 2 to be shrunk by a factor of three prior to the spacecraft encounter. The same tailward bias was observed in the astrometry of C/2013 A1 as the size of the synthetic aperture increased, therefore one should take the astrometric position corresponding to the extrapolation of the trend to a theoretical zero-sized aperture or at least to the brightness peak. However, we had no control nor information on most of the astrometry that was reported to the Minor Planet Center, some of which obtained by amateur observers. 7