Octant (instrument)
The octant, also called a reflecting quadrant, is a reflecting instrument used in navigation.
Not to be confused with Sextant, a similar type of reflecting instrument.Etymology[edit]
The name octant derives from the Latin octans meaning eighth part of a circle, because the instrument's arc is one eighth of a circle.
Reflecting quadrant derives from the instrument using mirrors to reflect the path of light to the observer and, in doing so, doubles the angle measured. This allows the instrument to use a one-eighth of a turn to measure a quarter-turn or quadrant.
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Demise of the octant[edit]
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 sometimes larger than 90°, and thus not possible to measure with an octant. For that reason, Admiral John Campbell, who conducted shipboard experiments with the lunar distance method, suggested a larger instrument and the sextant was developed.[10]
From that time onward, the sextant was the instrument that experienced significant development and improvements and was the instrument of choice for naval navigators. The octant continued to be produced well into the 19th century, though it was generally a less accurate and less expensive instrument. The lower price of the octant, including versions without telescope, made it a practical instrument for ships in the merchant and fishing fleets.
One common practice among navigators up to the late nineteenth century was to use both a sextant and an octant. The sextant was used with great care and only for lunars, while the octant was used for routine meridional altitude measurements of the Sun every day.[7] This protected the very accurate and pricier sextant, while using the more affordable octant where it performs well.
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Use and adjustment[edit]
Use and adjustment of the octant is essentially identical to the navigator's sextant.
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