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The Sextant
© Copyright 2001, Jim Loy
The sextant is a device used for navigation. It allows
you to measure the angle between the sun, or a star, and the horizon. Then,
with the aid of a clock, and perhaps a compass, you can find your position on a
map. The sextant (supposedly an improvement over the earlier quadrant) consists
of a weak telescope, a fixed mirror that covers the right side of your view
(allowing you to see the horizon to the left of the mirror), and an upper
mirror that rotates to give you a view of the sun or star. And there is a scale
that shows how much this mirror has rotated. The scale is fairly accurate, and
usually has a fine-adjustment knob. The fixed mirror is fixed so it allows you
a view of the upper mirror. The angle of rotation (from zero degrees) of the
upper mirror is half that of the angle of the sun or star above the horizon.
The sextant also has light filters, to protect your eye from the bright light
from the sun.
Addendum:
A sextant is called a sextant because its moving parts pivot about an
angle of 60 degrees, or 1/6 of a circle. Its scale goes from 0 to 120 degrees
(1/3 of a circle, by the way). The sextant was an improvement over the octant,
also called a quadrant. This instrument had a scale from 0 to 90 degrees (1/4
of a circle, thus quadrant), while its moving parts moved about an angle of 45
degrees (1/8 of a circle, thus octant). Other than that, the octant (or
quadrant) was the same instrument as the sextant. These instruments were mainly
improvements over older instruments because the navigator could measure angles
without moving his eye. Eye movement was a big source of error, because the
instrument may move while you move your eye, and because moving the eye may
actually change the angles (parallax), slightly.
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