6. Results for the solar diameter
The Rc values, from Table 1, which are plotted in Fig. 2, show that for two different observers using the same technique around the epoch 1680, some results are in considerable discord with one another. These measurements were made by Picard (1666) (ref nos. 3 and 5) and La Hire (1683) (ref nos. 6 and 8). This comparison shows that there are some difficulties in deducing a value for the solar diameter at this early period. The 30 Rc values listed in Table 1 (derived from 71000 measurements) considered as a unit lead to a mean value for the solar diameter (). If the very long series of observations (more than 10000 measurements or longer than 20 years with several observers, and a possible personal bias) or the very short sets (less than 90 measurements) are rejected, 17 sets remain. These are numbered in bold type in Table 1 and include in total 22000 individual measurements. The mean value for the solar diameter derived from this restricted data set is (). These two results are very close and taking into account the possible errors, the most probable value for the solar semi-diameter at 1 AU is . This value is the same as that given by Wittmann et al. (1981), and exceeds by Laclare's value (1993): . From the present data analysis, there is no evidence of any secular variations in the solar diameter when diffraction at the objective lens is taken into account. This result supports those of Morrison, Parkinson and Stephenson (1980 and 1988) which indicate that the solar diameter exhibits long-term stability. However, the findings of this investigation are in clear discord with those obtained by Eddy and Boornazian, and by Ribes et al. which indicate a major secular decrease in the solar diameter: the secular decrease from Eddy and Boornazian would bring to a variation of 7" after three centuries; as the apertures of 17th century refractors increased from 2.0 to 2.5 cm, the diffraction correction decreased by about 3".
© European Southern Observatory (ESO) 1997
Online publication: April 28, 1998