Data span and O−C residuals for comet C/1998 P1 Williams. Upper panel: time distribution of heliocentric (red dots) and geocentric distance (green dots) for all measurements that were taken into account to determine the orbit. The moment of perihelion passage and its heliocentric distance are indicated by vertical and horizontal dotted black lines, respectively. The following three panels show the time distribution of the O−C residuals for the respective orbital solutions, where residuals in right ascension are displayed by magenta dots and those in declination are represented by blue dots. Gray vertical bands show the area where residuals are below 1σ uncertainty of orbit determination. The asymmetric NG solution (preferred, second panel from the top) is compared with the symmetric NG solution (third panel) and pure GR solution (lowest panel).