Reflecting Instrument - Types of Sextants

Types of Sextants

While most people think of navigation when they hear the term sextant, the instrument has been used in other professions.

Navigator's sextant

The common type of instrument most people think of when they hear the term sextant.

Sounding sextants

These are sextants that were constructed for use horizontally rather than vertically and were developed for use in hydrographic surveys.

Surveyor's sextants

These were constructed for use exclusively on land for horizontal angular measurements. Instead of a handle on the frame, they had a socket to allow the attachment of a surveyor's Jacob's staff.

Box or pocket sextants

These are small sextants entirely contained within a metal case. First developed by Edward Troughton, they are usually all brass with most of the mechanical components inside the case. The telescope extends from an opening in the side. The index and other parts are completely covered when the case cover is slipped on. Popular with surveyors for their small size (typically only 6.5–8 cm in diameter and 5 cm deep), their accuracy was enabled by improvements in the dividing engines used to graduate the arcs. The arcs are so small that magnifiers are attached to allow them to be read.

In addition to these types, there are terms used for various sextants.

A pillar sextant can be either:

1. A double-frame sextant as patented by Edward Troughton in 1788.
2. A surveyor's sextant with a socket for a surveyor's staff (the pillar).

The former is the most common use of the term.