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How a Sextant Works

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Celestial Navigation Equipment and Techniques

Celestial Navigation has been used for many centuries. With accurate observations of heavenly bodies and the use of a Nautical Almanac, it is a reliable method of navigation for yatchspeople and vessels in the open ocean.

A sextant is a precision instrument for celestial navigation. The history of navigation at sea reveals a great number of devices for observing the horizon in relation to the elevation of the sun, moon and stars, their relative positions and changes in position as the year progressed. Recordings of such observations for every hour of every day of the year result in the Nautical Almanac.

Between the 16th and 19th centuries variations of a cross staff (two arms at 90°) were used for observations. The octant was then developed with its two arms placed at 45° (1/8 of a circle). A refinement of the octant was the sextant with its two arms placed at 60° (1/6 of a circle).

The main function of a sextant is to measure the angle of a heavenly body above the horizon. Use of a sextant takes practice. The basic procedure is to set the sextant reading on zero. Identify the heavenly body (e.g. star, sun or moon). Look through the telescope until the heavenly body appears in the mirror. Slowly lower the sextant, moving the index arm at the same time and keeping the star in sight in the mirror. Continue lowering until the horizon appears in the clear part of the glass. The reading is taken when the image of the heavenly body (brought down using the method above) is just touching the horizon.

A sextant may also be used for fixing position in coastal navigation. Here the sextant is turned horizontally and used to sight objects on the coastline.

Interested in buying a sextant. There are a lot of good resources for a new one, and equally many for a used one, but there is much you should know first:

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