Sextants

A sextant is a measuring instrument used to measure the angle of elevation of a celestial object above the horizon. Making this measurement is known as sighting the object or taking a sight. more...

The angle, and the time when it was measured, are 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.

The scale of a sextant has a length of 1/6 of a full circle; 60°, hence the sextant's name. An octant is a similar device with a shorter scale, 1/8 of a circle; 45°, which was in use until 1767 when it was quickly replaced by the sextant. In 1767 the first edition of the Nautical Almanac tabulated lunar distances, enabling navigators to find the current time from the angle between the sun and the moon. This angle is however sometimes larger than 90°, and thus not possible to measure with an octant.

Sir Isaac Newton invented the principle of the doubly reflecting navigation instrument, but never published it. Two men independently rediscovered the sextant around 1730: John Hadley (1682-1744), an English mathematician, and Thomas Godfrey (1704-1749), an American inventor. The sextant replaced the astrolabe as the main instrument for navigation.

Advantages

The specific feature that let the sextant displace the astrolabe is that celestial objects are measured relative to the horizon, rather than relative to the instrument. This allows much better precision.

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