It may here be mentioned that it was comparatively easy to
determine the latitude of a ship at sea every day when the sun
was visible. The latitude--that is, the distance of any spot
from the equator and the pole--might be found by a simple
observation with the sextant. The altitude of the sun at noon is
found, and by a short calculation the position of the ship can be
ascertained.
The sextant, which is the instrument universally used at sea, was
gradually evolved from similar instruments used from the earliest
times. The object of this instrument has always been to find the
angular distance between two bodies--that is to say, the angle
contained by two straight lines, drawn from those bodies to meet
in the observer's eye. The simplest instrument of this kind may
be well represented by a pair of compasses. If the hinge is held
to the eye, one leg pointed to the distant horizon, and the other
leg pointed to the sun, the position of the two legs will show
the angular distance of the sun from the horizon at the moment of
observation.
Until the end of the seventeenth century, the instrument used was
of this simple kind. It was generally a large quadrant, with one
or two bars moving on a hinge,--to all intents and purposes a
huge pair of compasses. The direction of the sight was fixed by
the use of a slit and a pointer, much as in the ordinary rifle.
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