We collected all astrometric observations of the periodic comets 31P/Schwassmann-Wachmann 2, 32P/Comas Solá, 37P/Forbes, and 43P/Wolf-Harrington and major part of data for 16P/Brooks 2 and 21P/Giacobini-Zinner. All these comets have been observed at more than seven apparitions. Observations for each apparition were selected according to the objective criteria elaborated by Bielicki & Sitarski (1991). The number of residuals for some overpopulated apparitions in the observational data were reduced by taking into account normal places (i.e. more than two observations on the same day are replaced by a single average comet position). The mean residuals computed for each apparition separately allowed to calculate the mean residual a priori for the whole observational set of each comet. The mean residual a priori should be compared with the RMS residual resulting from the numerical model of the comet's motion. The global characteristics of observations are presented in Table 2, and distribution of observations with respect to the perihelion passages and approaches of the comets to the Earth are presented graphically in Fig. 1.
Copyright ESO 2001