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Published in Splinter Robert, Illustrated Encyclopedia of Applied and Engineering Physics, 2017
[computational, general, geophysics] Instrument used by sailors in the late seventeenth and most of the eighteenth century for navigation at sea. The sextant can determine the angle between two points in a distant field, such as celestial objects and when the location of one of the objects is known as a function of time, the other object can provide the mechanism for determining the orientation as well as latitude and longitude of the location from which the measurement is made. The device uses two mirrors that can be used to merge (superposition) the images of two objects by adjusting the angle of the second mirror, yielding the angle of the second object (e.g., the sun) with respect to the first (i.e., the horizon). The accuracy of the sextant is 10 s, where a degree is subdivided in 60 minutes and a minute in angle is subdivided in 60 s (see Figure S.48).
Navigation systems and equipment
Published in Alexander Arnfinn Olsen, Core Principles of Maritime Navigation, 2023
The sextant is an instrument that measures the angles between two or more objects at sea. The sextant derives its name from the Latin for 1/6th (Sextans) as the sextant is shaped to form a sector, which is 60 degrees of 1/6 of a circle. The sector-shaped part is called the frame. A horizontal mirror is attached to the frame, along with the index mirror, shade glasses (sunshades), telescope, graduated scale, and a micrometre drum gauge. The normal graduations of the arc, to the left of zero, extending from 0 to 130 degrees are referred to as ON the arc. To the right of 0 degrees, the graduations extend for a few degrees and are referred to as OFF the arc. When reading OFF the arc, graduations of the micrometre should be read in the reverse direction (59 as 1’, 55 as 4’ and so on). The sextant works by applying the principle of double reflection. As a ray of light (incident ray) enters the frame, it is reflected off a plane mirror. The angle of the incident ray is equal to the angle of the reflected ray when the incident ray and reflected ray lie on the same plane. When a ray of light suffers two successive reflections in the same plane by two plane mirrors, the angle between the incident ray and the reflected ray becomes twice the angle between the mirrors. This means the sextant can measure angles of up to 120 degrees, though, in actual fact, because the arc of the sextant is a little over 60 degrees, the total angle measurable by the sextant is closer to 130 degrees. The sextant is a vital piece of navigation equipment and can be used by the ship’s navigator to determine the Vertical Sextant Angle (VSA), the Horizontal Sextant Angle (HSA), and altitudes. Using the sextant requires a good deal of training and experience, but once mastered, it is an excellent tool that can be used when electronic navigation equipment and systems fail.
Vertical Sextant Angles, tidal streams and co-tidal data
Published in Philip M. Smith, Tides, 2018
The sextant is a marine instrument designed to measure angles, generally vertical angles in conjunction with astronomical bodies in celestial navigation, to ascertain position lines and determine a ship’s position. It can be used both vertically to measure altitude above the horizon, and horizontally to determine the angle between terrestrial marks such as prominent lights or features marked on a chart used for navigational position fixing.
Safety first: The risk of over-reliance on technology in navigation
Published in Journal of Transportation Safety & Security, 2022
Jianjun Wu, James Thorne-Large, Pengfei Zhang
Therefore, over-reliance on a single mean of ship positioning method has great potential danger. For example, in crew training or simulator training, many trainees look too much at the Radar or ECDIS screen, rarely keep lookout by observing the real situation, and ignore the visual lookout, not to mention the skills of using classic navigational instruments. For example, sextant is a classic astronomical navigation instrument that people measure the angle between the sun or other celestial bodies and the sea level at a certain time, and then quickly get the geographical latitude of the ship position. Although radio navigation system and satellite navigation system have appeared one after another, which provides a more convenient and accurate solution for navigation, it is gradually becoming mor relevant to fix position by the traditional method due to the continuously emerging signal interference events. With its advantages of simplicity, reliability, good concealment, low price and no coverage area restrictions, the OOWs should be proficient in the use of sextant. Some other crews, meanwhile, are overconfident. For example, the captain of Cruise ship “Costa Concordia” trusted his experience and judgement more than the instruments and equipment on the bridge, forcibly modified the scheduled course, and made an error assessment until the ship ran aground. As an instance, in order to avoid the interference of the frequent alarm sound of modern navigation equipment, the crews completely disable the alarm function and ignore the main function of navigation equipment.
Instruments of authority: Tycho Brahe’s technological illustrations
Published in History and Technology, 2018
Using these letter labels, Tycho could talk the viewer through the process of using the instrument in question. For example, he describes the use of a sextant (Figure 1) to find the angular distance between two stars: The use of the instrument requires two observers. One of these puts his eye to the pinnule C and from there through its slits sights a star along the cylinder A. He then keeps the sextant fixed with the aid of the staffs at P. … The second observer then moves the alidade with its pinnule at D until he, too, can sight the other star on both sides of the cylinder A. This has to be tried to and fro until both observers at one and the same time each sights his star. Having called to each other they stop the observation, and the graduated arc of the sextant is turned on the spherical support towards the eye of one of the observers, so that he can conveniently read the distance between the two pinnules, which was what he sought.