3 Comparison with theoretical positions

We compared our complete set of measured positions with those calculated using the parameters given by Jacobson et al. (1991) (in their Table 5). The positions of Neptune were obtained from the ephemeris DE403 (Standish et al. 1995). These positions are related to J2000 reference system. However, the Jacobson parameters give rise to positions of Triton related to Neptune in the B1950.0 reference system. So we must transform the positions of the planet from J2000 to B1950.0 and, after we calculate the positions of the satellite, and convert these positions to J2000. To perform these convertions we used the procedure described in Aoki et al. (1983), Appendix 2. To add the E-Term of the aberration in the inverse transformation (J2000 to B1950.0) we used a process of successive approximations (Rapaport 1996).

Our positions (for all observations) have the following (O-C) residuals: $\overline{x}=$ $0{\hbox{$.\!\!^{\prime\prime}$}}014$, $\overline{y}= -0{\hbox{$.\!\!^{\prime\prime}$}}007$, $\sigma_x= 0{\hbox{$.\!\!^{\prime\prime}$}}086$and $\sigma_y= 0{\hbox{$.\!\!^{\prime\prime}$}}082$. These values are slightly better than those presented in Paper I, probably due to the fact that the images for each frame were obtained with an accurate determination of the good exposure time.

 

Figure 2: The (O-C) of Triton as function of time

In Fig. 2 we present the (O-C) of the positions of Triton related to Neptune as function of the observational dates. The photographic and the previous CCD observations are also plotted. We can see that the residuals are almost uniform for these 13 years. Some (O-C)_x values for 1997 are larger than $\overline{x}$ + 3$\sigma$ probably due to the bad meteorological conditions in the two considered nights.

The (O-C) for the positions of Triton related to Neptune as function of the longitude of the satellite for the observations of this paper (1995-1997) are presented in Fig. 3.

 

Figure 3: Residuals for Triton referred to Neptune versus the longitude

We can see that the distribution of the residuals is approximately uniform.