The instrument lying on the plane allowed an observer to obtain
the height of the sun without having to point directly to it.
The basic concept was that of a gnomon. The problem of
vertical shadows is that the length of the shadow projected by
the gnomon is proportional to the co-tangent of the angular
height, which introduces a non-linearity to the scale.
Pedro Nunes devised this solution in order to convert the reading
of the angle into a circular scale.
The base of the instrument could be a wood plate. In this base a
circle, and a tangent to this circle, were drawn. Then, a
triangular plate made of wood or brass would be attached to the
base in the position seen in the figure:
The height (altura) of the sun, that is the shadow abc
(or alternatively the zenith distance), could be read in the
circular scale cd.
This instrument was in fact used aboard by Joćo de Castro in a
expedition to the Red Sea. In spite of the good readings
obtained by Castro, this instrument was not welcome among mariners.
The main reason was that, in order to give good readings, the
instrument should lay on a stable, horizontal surface, which was
only possible in calm days or when in land.