octant

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octant, a reflecting navigational instrument for measuring the altitude of heavenly bodies, having an arc of one-eighth of a circle, but, because of its reflecting properties, capable of measuring altitudes of up to 90°. Hadley's reflecting quadrant, which began replacing Davis's quadrant in the 1730s, was in fact an octant, but was still called a quadrant because that was the name by which all seamen called the existing seaman's quadrant. Octants remained in use by navigators up to the 19th century. Sextants, which were introduced during the second half of the 18th century, were generally used for lunar distance in the calculation of longitude. When the introduction of chronometers made lunar calculations no longer necessary, the sextant replaced the octant as the standard navigational instrument for the measurement of altitudes.

octant eighth part of a circle; (spec. astron.) point 45° (i.e. ⅛ of 360°) distant from another. XVII. — L. octāns, -ant- half quadrant, f. octō EIGHT.

octant n. an obsolete instrument in the form of a graduated eighth of a circle, used in astronomy and navigation.

octant See octree.

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