Sextant
A sextant is an instrument for measuring the altitude of a celestial body above the earth's horizon in terms of the included angle. By noting the exact time of such an observation and by referring to the Nautical Almanac, the line of position of the observer can be established. Two or more such observations of different celestial bodies, or of the same body in different positions, lead to a definite "fix" of position; i.e., the latitude and longitude of the position. A sextant can also be used for measuring horizontal angles.
The sextant derives its name from the fact that it originally had the form of a sixth part of a circle; however, the name has lost much of its original meaning since sextants are now constructed of various arcs of the circle. Although the actual arc of the conventional sextant is only approximately 60 degrees, angles of approximately 120 degrees can be measured, by virtue of the geometrical arrangement of the mechanism.
The basic parts of a marine sextant are: main frame, on which all the parts are mounted; limb, on which graduations in degrees are marked; index arm, mounted in such a way as to pivot about the exact center of curvature of the limb; index mirror, mounted at the upper end of the index arm and perpendicular to the plane of the limb; horizon glass, mounted on the frame and parallel to the index mirror at zero reading of the sextant (one half of this glass is silvered as a mirror, while the other half is clear glass); telescope, attached to the frame, to magnify the horizon as seen through the horizon glass; and a micrometer drum or vernier readout. On older types, the vernier is read directly in connection with the main scale, there being no micrometer drum.
The telescope is first focused by observing the horizon. Holding the sextant as nearly vertical as possible, it is faced toward the celestial body for which the altitude is desired and the image of this body is brought into the mirrored portion of the horizon glass by moving the index arm along the limb of the sextant. The bottom of the image is brought in line with the horizon, viewed in the clear glass alongside the mirrored glass, by moving the index arm. As soon as the image is on the horizon line, with the sextant vertical, the precise time of the observation is recorded with the reading on the sextant scale. This reading is often called the sextant altitude and is subject to one or more of the following corrections for error: sextant index correction, refraction by the earth's atmosphere, dip of the horizon, semidiameter (use of edge of celestial body, instead of center), parallax (difference in direction of celestial body when viewed from earth's surface as compared with earth's center), and coriolis (an error due to the earth's rotation). Data for making these corrections, with the exception of sextant index correction, are available in navigation almanacs. Stanley London's most popular model of sextants include the 3 inch Polished, 4 inch Antique, Captain's, and 6 inch Antique sextants.
