A Sextant is an instrument used to measure the angle between any two visible objects. such measurements can then be used to determine the observer's geographical position or for other navigational, surveying, or astronomical applications.It was developed independently and at the same time by John Hadley in England and by Thomas Godfrey in Philadelphia, about 1731. Its construction is based on the principle that a reflected ray of light leaves a plane surface at the same angle at which the direct ray strikes the surface. The sextant consists of an arc of a circle, marked off in degrees, and a movable radial arm pivoted at the center of the circle. A movable mirror is fixed at the end of the arm. The other end extends to the scale. A telescope is mounted on the sextant, and a glass mirror is mounted in front of the telescope.

To operate the sextant, the operator looks through the telescope straight at the horizon. And then moving the mirror the sun or any particular star made to appear exactly on the horizon. The arm, which moves the mirrors, gives the required measurement of the angle. From this angle and the exact time of day, the latitude is determined by means of published tables.



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