Abstract
The phenomenon called the ``sub-Sun'' is the specular reflection of sunlight by horizontally oriented plates of ice. Although well-known in meteorological optics, the hydrodynamics of the orientation is not quantitatively understood. I review the theory of torques on objects at low Reynolds numbers, define coefficients
C
o
,
C
p
, and
C
ψ
which describe the orienting torques on discs, rods, and hexagonal prisms, and report here the results of experiments to measure
C
o
and
C
p
.