Marine Glossary

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Sextant

A device used to measure the angular heights of celestial bodies above the horizon. It can also be used for coastal piloting to find distance off from the angular heights of landmarks with charted elevations. The inherent precision of top-line metal models (costing $800 to $3,000) is some 0.1' to 0.2', but this does not mean any single sight will be this accurate. Even highly experienced navigators will get variations of up to 1.0' in a series of nominally identical sights. Consequently, to approach a fix accuracy equivalent to the quality of a good sextant, several sights must be taken and then averaged in a special manner that accounts for the motion of the boat.

Plastic sextants (Davis Mark 15 and Mark 25, or the no-longer-made Ebbco) have a lower inherent precision, 1' to 2', with even higher variations within a series of sights. Nevertheless, celestial fixes to within 3 or 4 miles accuracy can still be achieved with one of these plastic sextants by averaging many sights and checking the index correction frequently. See our discussion in our textbook or our book on plastic sextants. See also our Glossary entry for the Mark 3 sextant.

The sextant was invented by Sir Issac Newton in the mid 1700's. Early models used a vernier scale, which was replaced with more convenient micrometer drums some time in the 30's. Important features to look for are large mirrors, a wide, unobstructed field of view, with lighted arc and micrometer. An optimum all-purpose telescope is about a 4x40. The most popular sextant worldwide is the Astra IIIb, which marks a peak in performance per dollar. These are made in China. Top quality metal sextants are made also by Freiberger (may be out of business, was originally East Germany), Plath (Germany), and Tamaya (Japan). See Sextant Sights, Accuracy, Vernier, Index Correction, and Side Error.

Also this new note from March 2020. We have just learned that the Tamaya 4-40 scope will not focus close enough to use with an artificial horizon. We have to learn more of this, but something to keep in mind if you have that intention. This does not seem to be a problem with the scope on the Chinese sextants, which we have used for years with artificial (and real) horizons.