How a Sextant
There's nothing mystical
or complicated about a sextant. All it is is a device that measures the
angle between two objects.
The sextant makes use of two mirrors.
With this sextant, one of the mirrors ( mirror A in the diagram) is
half-silvered, which allows some light to pass through. In navigating, you
look at the horizon through this mirror.
The other mirror (mirror B
in the diagram) is attached to a movable arm. Light from an object, let's
say the sun, reflects off this mirror. The arm can be moved to a position
where the sun's reflection off the mirror also reflects off mirror A and
through the eyepiece. What you see when this happens is one object (the
sun) superimposed on the other (the horizon). The angle between the two
objects is then read off the scale.
What makes a sextant so useful
in navigation is its accuracy. It can measure an angle with precision to
the nearest ten seconds. (A degree is divided into 60 minutes; a minute is
divided into 60 seconds.)