A precision navigational instrument falling rapidly into disuse
A sextant is an instrument used to measure the angle between any two visible objects.
Its primary use is to determine the angle between a celestial object and the horizon – known as the altitude of object.
Making this measurement is known as taking a sight or shooting the object.
The angle measured, combined with the time when the measurement was made, can be used to calculate a position line on a nautical chart.
By shooting two, three, or four celestial objects, one’s position can be determined with a high level of accuracy.
The scale of the sextant is 60°, or one-sixth of a circle.
Sextans is Latin for “one-sixth”.
The sextant was developed as an improvement over the octant, which had a scale of 45°, or one-eighth of a circle.
In addition to the frame or body and the scale or arc inscribed on the limb, a sextant has an index arm, which moves the index mirror.
Modern index mirrors are 5 cm in diameter, while older ones were about half that size.
Traditional sextants have a half-horizon mirror, dividing the field of view in two (on one side the horizon is observed and on the other side the celestial object is observed).
Many newer sextants have whole-horizon mirrors, which is half-silvered to provide a full view of the horizon.
Both work well and the choice of which to use is largely personal to the navigator.
Sextants also have filters for use when shooting the sun or to reduce the effects of haze.
Most sextants mount a low-power monocular to improve viewing, but many navigators prefer a simple sighting tube with no magnification.
With a worm adjustment and vernier, sextants can be read to within 0.2 minutes.
Under good observation conditions, an experienced navigator can determine the ship’s position with an accuracy of about 0.25 nautical miles, but accuracy within two miles is more common and generally is more than acceptable when the ship is out of sight of land.
The major advantage of the sextant is that, combined with a accurate time-piece, it is wholly independent of electricity or modern technology.
It cannot be turned off by budget cuts and it cannot be jammed.