Published in Appl. Opt. 54, B1-B7 in 2015
G.P. Können, J. Tinbergen, and P. Stammes
Sirius was spotted with the naked eye at
broad daylight by looking along the finder of a 1 m telescope on La Palma Observatory
at a 2370 m
height. Sun elevation was 73°; Sirius was nearly straight under the Sun at 37°
elevation. The sky radiance, although not recorded directly, could be
determined from the simultaneously obtained high-precision wavelength-dependent
sky polarization data near Sirius. This was done by fitting the polarization
data with the doubling-adding KNMI (DAK) radiative transfer model, which
provided the values of the surface albedo and of the aerosol optical thickness
required for determining the absolute sky radiance. Our analysis implies that
Sirius, when positioned overhead, can be a daytime naked eye object from sea
level even if its culmination occurs at solar noon. It also suggests that the
second-brightest star (Canopus),
if positioned overhead, could be perceptible even at solar noon.