Are these sextants, or are they octants?

A Sextant

A sextant has a scale that reads to at least 120 degrees, and often a bit extra. The octant (originally named the "quadrant", which adds to the confusion) was the predecessor of the sextant. An octant scale usually reads to at least 90 degrees, with the scale frequently extended to 100 degrees or so. An octant was intended for measuring altitudes above the horizon, so an angular range to 90 degrees was all that was necessary and a measurement to within a few minutes of arc was usually adequate for navigation.

Measurement of the "lunar distance", the angle-in-the-sky between the Moon and the Sun or a star or planet, however, was much more demanding. An accuracy of better than one minute of arc was required to limit the error in longitude within 30 minutes (30 miles, near the equator). For this purpose, a brass instrument was considered necessary because wood did not provide the necessary long-term dimensional stability. Lunar-distance angles could usefully be measured up to 120 degrees and sometimes more. (For this purpose, the quintant, measuring to 144 degrees, was introduced in the late 19th century.)

An Octant

So instruments diverged into two classes, generally speaking: cheap wooden octants measuring to about 90 degrees and expensive brass sextants to about 120. However, some ebony-and-ivory instruments were available which could measure up to 120 degrees, but I suspect that they were unsatisfactory for lunar-distance measurement.

When a navigator possessed an accurate metal sextant, he might well reserve that precious device for use in measuring lunar distances, and bring out his wooden octant for the simpler task of measuring altitudes above the horizon.

When the chronometer eventually became cheap enough to be available to the ordinary mariner, lunar distances became obsolete (except for those careful souls who liked to make a cross-check on their chronometer). So the old wooden octants remained in use by some navigators, well into the 19th century, doing all that was then needed.

George Huxtable

Sextant Discussion Group