The sextant is an instrument used to measure the elevation angle of a celestial body above the horizon. Technically, the measurement is done by making the celestial body and the horizon line up. The date and angle are used to calculate a specific position on an air or nautical chart; the sextant is also used to calculate the latitude by sighting the sun at noon. The sextant derives its name from the fact that its scale has a length of one sixth of a full circle, equal to 60°.
The sextant works by the principle of double reflection: if a light beam hits the same surface twice, the deviation angle is twice the angle between the reflectors. The sextant arc scale is 60° but it’s doubly graduated, so it is possible to read directly the double angle formed by the two mirrors. It was Sir Isaac Newton the inventor of the double reflection principle in nautical instruments, but these researches were never published.
Use of the sextant
Sighting through a sextant makes two points line up. One is the sky or any celestial body visible on the index mirror, the other one is the horizon on the horizon mirror. By a proper adjustment, the image of the celestial body is brought into line with the horizon. The angle reading and the time and date recording are made simultaneously. Then, the elevation angle – or vernier – of the graduated arc is taken and is recorded along with the date. The date and time are used to subtract the data of the celestial body from the ephemeris, and are useful for calculation. The measurement is then used to determine a position through several mathematical calculations. The simplest method consists in drawing the elevation of the sighted body on a globe. The intersection between two or more elevation circles, referred to two or more celestial bodies, provides a precise localisation.
A closer look to the sextant
The index arm or “alidade” moves the index mirror. The indicator, or “line of faith” points at the arch to show measurement. The horizon mirror is integral with the frame and is of various types. The frame connects all the parts. The telescope is integral with the frame and is opposite the horizon mirror. In short, the sextant has a half-horizon mirror which divides the field of view in two. The outer part is clear and lets the viewer sight the horizon; the inside is reflective and lets the viewer sight the celestial body, which is also reflected by the index mirror. The advantage is that both the horizon and the celestial body are bright and thus more easily visible. It is especially used during the day, in the sunlight or at dusk, with the stars, when the horizon is more visible. However, it is necessary to make sure that the lower part of the celestial body is lined up with the horizon.
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Professional sextants can measure up to 1 minute of a degree, equal to 1/60 degree. With more precise ones you can make measurements through a verner up to 0,2 minutes of a degree. Since a minute degree error corresponds to a nautical mile, the best precision one can obtain by celestial navigation is 0,1 nautical miles, equal to about 200 yards (186m). Temperature changes can affect the shape of the sextant’s arc, and thus cause errors. Many sailors keep it in a watertight case in order to protect it from temperature fluctuations. A standard frame can usually stabilize errors caused by temperature fluctuations. The adjustment knobs are separated from arc and frame in order to prevent them from damage. The sextants used at the tropics are often painted in white in order to reflect the sunlight and keep cool. The high precision sextants are built in a special alloy called invar which has a low thermal expansion coefficient. Special quartz or ceramic sextants have also been built, which have an extremely low thermal expansion coefficient. Brass has a lower thermal expansion coefficient than aluminium, but the latter is lighter and easier to use.