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Sextants
A sextant is an instrument generally used to measure the altitude of a celestial object above the horizon. Making this measurement is known as sighting the object, shooting the object, or taking a sight. more...
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The angle, and the time when it was measured, can be used to calculate a position line on a nautical or aeronautical chart. A common use of the sextant is to sight the sun at noon to find one's latitude. See celestial navigation for more discussion. Held horizontally, the sextant can be used to measure the angle between any two objects, such as between two lighthouses, which will, similarly, allow for calculation of a line of position on a chart.
The scale of a sextant has a length of 1⁄6 of a full circle (60°); hence the sextant's name (sextāns, -antis is the Latin word for \"one sixth\", \"εξάντας\" in Greek). An octant is a similar device with a shorter scale (1⁄8 of a circle, or 45°), whereas a quintant (1⁄5, or 72°) and a quadrant (1⁄4, or 90°) have longer scales.
Sir Isaac Newton (1643-1727) invented the principle of the doubly reflecting navigation instrument (a reflecting quadrant - see Octant (instrument)), but never published it. Two men independently developed the octant around 1730: John Hadley (1682-1744), an English mathematician, and Thomas Godfrey (1704-1749), an optician in Philadelphia. The octant and later the sextant, replaced the Davis quadrant as the main instrument for navigation.
Navigational Sextants
This section discusses navigator's sextants. Most of what is said about these specific sextants applies equally to other types of sextants. Navigator's sextants were primarily used for celestial navigation.
Advantages
Like the Davis quadrant (also called backstaff), the sextant allows celestial objects to be measured relative to the horizon, rather than relative to the instrument. This allows excellent precision. However, unlike the backstaff, the sextant allows direct observations of stars. This permits the use of the sextant at night when a backstaff is difficult to use. For solar observations, filters allow direct observation of the sun.
Since the measurement is relative to the horizon, the measuring pointer is a beam of light that reaches to the horizon. The measurement is thus limited by the angular accuracy of the instrument and not the sine error of the length of an alidade, as it is in a mariner's astrolabe or similar older instrument.
Read more at Wikipedia.org
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