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The octant derives its name from the fact that its scale consists of the eighth part of a circle, equal to 45° (1/8 of a circle). The reflecting octant was introduced by John Hadley (1682-1744) in 1731. Its use consists in the double reflection of the sun on two mirrors before this reaches the observer’s eyes.  The observer can see on the index mirror both the horizon and the doubly reflected sun image simultaneously. 



Despite the roll of the ship, it was possible to maintain a celestial body lined up with the horizon, which had previously been brought down to the latter by reflection; by so doing, it was possible to eliminate the causes of error during surveys taking place at sea, with instruments such as nautical astrolabes or quadrants. This device was used until 1767: in that year the first edition of the nautical almanac with the moon positions was published, which allowed sailors to find the current date in relation to the angle between moon and sun. However, this angle sometimes exceeded 90°, often making it impossible to use the octant; this paved the way for the use of the sextant.


The structure of the octant
The frame is in ebony, the goniometric scale is in ivory and is encased into the graduated frame; the alidade is in brass, and it has a reading screw lock and micrometric reading regulation screw; it is provided with round support pods on the back of the instrument.


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