Nautical instruments

Nautical instrumentsThe sextant is a nautical navigational instrument for measuring the angle between two objects by bringing into coincidence at the eye of the observer rays of light received directly from one object, and by reflection from the other, the measure being afforded by the inclination of the reflecting surfaces to each other. The sextant is especially used for measuring the altitudes of celestial bodies above the visible horizon. It consists essentially of the following main parts:

(1) An are, or limb (A), of approximately one sixth of a circle, from which the sextant takes its name. This limb is provided with a silver scale graduated to read degrees, each degree being subdivided into smaller divisions, which usually represent 20' of arc.

(2) An index arm (B), arranged to pivot about the exact center of curvature of the limb. Its lower end is provided with a vernier which moves along the scale of the limb, and the zero mark of which serves as the reading mark, or "index," of the sextant. The upper end of the index arm is fitted with a mirror (C) of silvered plate glass, called the index mirror. The plane of the index mirror is perpendicular to the plane of the limb of the sextant.

(3) A horizon glass (E), mounted upon the frame of the sextant. The half of this glass next to the frame is silvered as a mirror, the other half is clear glass. The horizon glass is adjusted so that its plane is perpendicular to the plane of the limb of the sextant, and parallel to the plane of the index mirror, when the index arm is at zero.

(4) A telescope (D), to direct the line of sight of the observer to the horizon glass in a line parallel to the plane of the limb, and to magnify observed objects.

The function of these four essential parts is as follows:

Suppose the index arm to be revolved to the right of the position shown on the picture, until the index mirror (C), is parallel to the plane of the horizon glass (E). In this position the zero mark of the vernier should be in line with the zero mark of the limb. With the index arm in this position, suppose the sextant to be held vertically, with the telescope directed at the horizon.

The horizon is far enough away from the observer so that the rays of light from it to the horizon glass and to the index mirror are sensibly parallel. The observer, looking through the clear half of the horizon glass, will see the horizon directly, and looking into the mirrored half of the horizon glass, will see the horizon reflected from the index glass.