Two calculated orbits for Phoebe are considered. Those calculated by
Jacobson (1998a) and Bec-Borsenberger & Rocher (1982). The comparison
of our observations with the calculated positions are presented in
Table 3. The mean of the observed minus calculated residuals are comparable
for both the considered orbits. The larger standard deviation relatively to the Bec-Borsenberger & Rocher orbit is due to a poor fitting
of the orbit which does not consider the observations made after 1981.
Jacobson | Bec-Borsenberger & Rocher | |||
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Satellite |
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Phoebe | -0.08 | 0.29 | -0.07 | 0.37 |
(0.14) | (0.26) | (0.92) | (0.33) |
In Fig. 4 is presented the (O-C) distribution of the residuals between these observations and Jacobson's theorethical positions, with respect to the date of the observation. Figure 5 concerns the same residuals, this time relatively to the true anomaly. Comparing these figures it is noticed that the (O-C) in the y direction are clearly positive and the largest values of the residuals are reached near the periapsis.
In order to understand the bias in the y direction we compared our statistics
(means and standard deviations, from Table 3) with those presented in
Table 3, by Jacobson (1998b). Considering the three best fits
(Wipple 1992,
1996 and Mulholland 1975, in Jacobson Table 3) it appears that the
statistics in the x direction are similar and we have the same result
for .
However, our mean in the y direction is significantly larger.
In Fig. 4 it is seen that the main contribution for the y mean is
given by two of the three missions in 1996. Furthermore, Fig. 5 shows
that the set nearest to the periapsis has its residuals larger than
0
5, and (see histogram in Fig. 1) they contribute with more than one
third for the number of 1996 observations. Therefore, this suggests that
the error in the y direction come, at least partially, from a bad fit
of the Phoebe orbit in the periapsis.
Copyright The European Southern Observatory (ESO)