Frenzie Mae Rivera BSMT – 2ANNavigation 3 August 23, 2010

THE SEXTANT

The name comes from the Latin sextus, or “one-sixth,” for the sextant’s arc spans 60°, or one-sixth of a circle. Octants, with 45° arcs, were first used to calculate latitude. Sextants were first developed with wider arcs for calculating longitude from lunar observations, and they replaced octants by the second half of the 18th century.

This is the instrument used for determining the angle between the horizon and a celestial body such as the Sun, the Moon, or a star, used in celestial navigation to determine latitude and longitude. The device consists of an arc of a circle, marked off in degrees, and a movable radial arm pivoted at the centre of the circle. A telescope, mounted rigidly to the framework, is lined up with the horizon. The radial arm, on which a mirror is mounted, is moved until the star is reflected into a half-silvered mirror in line with the telescope and appears, through the telescope, to coincide with the horizon. The angular distance of the star above the horizon is then read from the graduated arc of the sextant. From this angle and the exact time of day as registered by a chronometer, the latitude can be determined (within a few hundred meters) by means of published tables.

Graduated Arc - Indicates the number of degrees of an angle. Index arm – Pivots at one end to allow the attached index mirror to reflect an object onto the horizon glass and swings along

the arc scale on the other end to indicate what the angle measures. It is a type of ruler that determines direction or measures an angle.

Micrometer drum – Rotates to make fine adjustments when measuring angles and indicates minutes of a degree of angle. It is attached to the lower end of the index arm. One complete rotation moves the index arm 1° along the arc scale. The drum has 60 graduations, each representing1' of arc.

Vernier scale – Indicates tenths of a degree of angle. It is attached on the index arm adjacent to the micrometer drum and has 10 graduations, each representing 0.1’ of arc.

Index mirror – large polished plate that Reflects objects onto the horizon glass. Horizon glass – Allows the observer to view one object directly on one side while observing a second object reflected next to

it. The half of the horizon glass next to the frame is silvered to make that portion of the glass a mirror; the other half is clear glass. It is a small polished glass plate that reflects light.

Telescope – Directs the line of sight of the observer to the horizon glass and magnifies the objects observed. Telescope clamp - reinforcing circle. Eyepiece - lens the user looks through. Telescope printing - lens adjustment. Filters – a colored transparent substance that protects the observer’s eyes when viewing the Sun. Release levers – Disengages the index arm from the arc scale to allow the index arm to move freely. It holds the sextant in

place. Frame - structure that serves as the base for the different parts of the sextant. Screw to regulate small mirror - piece of metal used to adjust the horizon mirror.