horizon, the accuracy of the measurement will remain high compared to the magnitude of the movement. The sextant is not dependent upon electricity (unlike many forms of modern navigation) or for that matter anything reliant on human-controlled signals (such as GPS satellites). For these reasons it is considered to be an eminently practical back-up navigation tool for ships. Design The frame of a sextant is in the shape of a sector which is approximately of a circle (60°), hence its name (sextāns, sextantis is the Latin word for "one sixth"). Both smaller and larger instruments are (or were) in use: the octant, quintant (or pentant) and the (doubly reflecting) quadrant span sectors of approximately of a circle (45°), of a circle (72°) and of a circle (90°), respectively. All of these instruments may be termed "sextants". Attached to the frame are the "horizon mirror", an index arm which moves the index mirror, a sighting telescope, sun shades, a graduated scale and a micrometer drum gauge for accurate measurements. The scale must be graduated so that the marked degree divisions register twice the angle through which the index arm turns. The scales of the octant, sextant, quintant and quadrant are graduated from below zero to 90°, 120°, 140° and 180° respectively. For example, the sextant illustrated has a scale graduated from −10° to 142°, which is basically a quintant: the frame is a sector of a circle subtending an angle of 76° at the pivot of the index arm. The necessity for the doubled scale reading follows from consideration of the relations of the fixed ray (between the mirrors), the object ray (from the sighted object) and the direction of the normal perpendicular to the index mirror. When the index arm moves by an angle, say 20°, the angle between the fixed ray and the normal also increases by 20°. But the angle of incidence equals the angle of reflection so the angle between the object ray and the normal must also increase by 20°. The angle between the fixed ray and the object ray must therefore increase by 40°. This is the case shown in the graphic. There are two types of horizon mirrors on the market today. Both types give good results. Traditional sextants have a half-horizon mirror, which divides the field of view in two. On one side, there is a view of the horizon; on the other