| definition |
An instrument used mainly in nautical astronomy to determine the angular distances
of celestial bodies above the horizon. The sextant consists of a 60° graduated arc,
or limb, a small telescope, and two mirrors. Only half of one of the mirrors, the
horizon glass, is silvered and, like the telescope, it is fastened to the frame supporting
the limb. The other mirror, the index mirror, moves with an index arm pivoted at the
center of the arc. The index arm is equipped with a vernier which moves along the
limb. An object in direction S can be observed in the telescope through the un-silvered
portion of the horizon glass. By moving the index arm, the second object S' is made
to coincide with S in the telescope. According to the law of reflection, the angle
between S and S' is double the angle between the mirrors. The angular distance between
the objects can therefore be obtained from the measurement of angles between the planes
of the mirrors.
|
|
