On 1987 March 29, another hybrid solar eclipse was viewed from Gabon where it only was a broken annular. After its beginning at sunrise in Argentina as an annular, the eclipse path crossed the Southern Atlantic Ocean where it briefly became total before reaching Gabon as an annular again.
Small groups had the chance to observe this remarkable eclipse.
In the Sky & Telescope issue of July 1987, this eclipse was descibed as barely total very briefly. This isn’t the case at all, the transition zone was in fact quite far from the coast of Gabon to the southwest in the Southern Atlantic Ocean. This eclipse wasn’t even annular in Gabon, it was a broken annular because photospheric beads remained visible at all times. Nevertheless the study of such unique eclipses can provide a wealth of information to obtain a reliable measurement of the true photospheric solar radius, plus the variability of the solar radius can be evaluated as well. The accurate determination of the solar photospheric radius has been an important problem in astronomy for many centuries. Moreover an accurate value for total solar irradiance (TSI) is crucial for the Sun-Earth connection, and represents another solar astrophysical fundamental parameter.
As one can see below there is a very good match between the eclipse simulation and the photographs taken through a thin cloud layer. It is also clear that the value of the photospheric solar radius needs to be amended. Indeed with the canonical value of 959.63 arc-seconds (Auwers, 1891) it is impossible to obtain a reasonably good match. A value around 960.00 arc-seconds at one astronomical unit is more appropriate.