An instrument used for navigation, which was invented in
1730 and superceded the astrolabe. A fixed
telescope is pointed at the horizon, and a radial arm is moved against an
arc graduated in degrees until a mirror which it bears reflects an image
of a known star or the Sun down the telescope to coincide with the image
of the horizon. The angular elevation of the star, with the exact time (see
chronometer) gives the latitude.
The air sextant is a similar instrument, usually periscope,
designed for use in aircraft, and has an artificial horizon, generally a
|A simple sextant as used for determining the altitude
of the Sun or a star above the horizon. The horizon is viewed through
the telescope while the image of the Sun viewed in the horizon and
index mirrors is made to coincide with the horizon by rotating the
index mirror. The altitude of the Sun can then be read off the scale.
Practical instruments incorporate a variety of filters to protect
the eyes of the user.