3. Corrections to measurements
The measures analysed here, considering the horizontal diameter, have been corrected for various factors (the vertical diameter is more affected by refraction). The main corrections (which values greater than 0".1), are given in Table 2; these relate to atmospheric refraction and turbulence, personal equation of the observer and diffraction at the objective. In particular, this last correction could be very important for instruments having a small objective - such as those employed during the 17th and 18th centuries. By the 19th century, instruments have larger diameters so that diffraction would not be so important. More than a century ago, André (1876, 1877) studied this aspect. Allowing for refraction and diffraction, the corrected value of the semi-diameter of the Sun at unit distance (Rc) can written in the form , where Ref is the correction for refraction and is the correction for diffraction. The diffraction correction, of the order of , corresponds to (red filter) or (green filter), where D is the diameter of the objective in cm. When D is not known, I have used an empirical formula ; this is valid up to 1870. After 1870, the diameter is considered to be 20 to 25 cm. The different values of are summarised in Table 2 and are listed in column 9 of Table 1 following the estimated value of D in column 8. As seen from this table, for the heliometer and astrolabe when observing two limbs of the Sun, the assumed correction ranges from about +1" to -0.3". The refraction correction Ref (equal to +0".3 for the semi-diameter), is applied to all R1 values deduced from the observations made up to the end of the 18th century. It is assumed that during the 19th and 20th centuries, the necessary correction was properly applied.
© European Southern Observatory (ESO) 1997
Online publication: April 28, 1998