The legs and furniture are brass. The handle is made of a dark hardwood walnut? The A certificate from the National Physical Laboratory is attached to the interior of the lid of the case, describing the corrections needed for the precise measurements, as seen in the certificate image. A small bent wire asdjusting wrench is also present, seen just below the eyepiece filter. Press tabs are provided to lock the hooks holding the case securely closed. Those promoting the use of lunar distances, or "lunars," for finding longitude at the end of the 18th century stimulated the invention of the sextant.
The sextant was designed to find longitude by measuring the angular distance between the moon and a nearby star. The octant, already invented in , could only measure up to 90 degrees of angle. The new "lunar method" for finding longitude, however, required the ability to measure angles up to degrees.
It is called a sextant because its arc spans 60 degrees, or one sixth of a circle, but the double reflection of the image of the celestial body enables the sextant to measure angles up to degrees. Like the octant, the sextant was used to measure the altitude of a body sun, moon, stars, planets above the horizon, as well as the angular distance between two bodies.
Because of its ability to measure larger angles the sextant gradually replaced the octant. Most have telescopic sights. In King Charles founded the Royal Observatory so astronomers could take precise measurements of the orbit of the moon in relation to fixed stars. The hope was that once the movements of the celestial bodies were properly catalogued, longitude could be found by taking three simultaneous observations of the distance between the moon and the sun or a fixed star, the altitude of the moon, and the altitude of the sun or star.
Used since ancient Egyptian time, the modern form of this instrument consists of a telescope that only moves vertically. A dial on one side, graduated from 0 to 90 degrees, is used to measure the inclination of the telescope.
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