Also called a box sextant. Edward Troughton invented this unique instrument in the early 17th century. It opens up with the cover that becomes a grip for taking readings. To take the image, a little scope pulls out from the side. A filter also swings out, when not needed it folds back inside. The readings are read of the scale with the tiny adjustable magnifying glass on the top. It is 8 cm in diameter and 4 cm deep with the cover under the top. This unique and unusual instrument comes with a wooden box.

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Sextants - Instructions on how to use a sextant©

Sextant, optical instrument used for the measurement of angular distance between any two objects. The instrument was invented independently, in about 1730, by both the english mathematician John Hadley and the American inventor Thomas Godfrey. the Chief use of the Sextant is in navigation: the sextant enables a navigator to measure the angular elevation of the sun and other celestial bodies and from this information the navigator's latitude and longitude-in other words, the navigator's location-can be determined.

The operation of the sextant depends upon superimposition of the images of the two objects whose distance is being measured. this is achieved by means of an optical system consisting of a telescope and two mirrors, one fixed and one movable. In the diagram shown below, the telescope t is mounted in a fixed position on the body of the instrument, pointing toward the mirror a. the top half of this mirror is transparent, and the bottom half is silvered. a second mirror, mirror b, is angled above mirror a. an observer looking through the telescope toward the horizon h sees the horizon through the un unsilvered portion of mirror a and at the same time sees the image of the star or the sun s on the silvered portion of mirror a, as re-reflected from mirror b above. by moving b by manipulating the lever l, the image of the star is brought into coincidence with the image of the horizon. The angular distance between the star and the horizon can then be read on a scale, which is engraved on the body of the sextant. This scale is an arc of one-sixth of a circle, or 60 degrees. each degree on the scale of the sextant is equivalent to two degrees of angular distance between the objects actually observed because the light from s reflects off two mirrors.

 When observations with sextants are taken aboard ships at sea, the actual observed horizon can be used for measuring altitudes. on land this method of observation is seldom possible because of the irregularity of terrain. In this case a so-called artificial horizon is employed, consisting of a pool of mercury or some other horizontal reflecting surface. by observing both the star itself and the image of the star in the mercury, a sextant reading can be obtained that is equal to double the actual altitude of the star.

A special type of sextant is the bubble sextant. In aircraft and on ships in a rough sea, when the visible horizon is not a clearly defined line, bubble sextants or bubble octants are used. the octant is an instrument that resembles a sextant but has an arc of only one-eighth of a circle, or 45 degrees. In these bubble instruments, the bubble of a built-in spirit level is observed in place of the horizon.

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