The Sextant What it is and how it works The parts of a Sextant What is a Sextant? A sextant is a measuring instrument generally used to measure the angle of elevation of a celestial object above the horizon Making this measurement is known as sighting the object, shooting the object or taking a sight The angle, and the time when it was measured, can be used to calculate a position line on a nautical chart What is a Sextant? Held horizontally, the sextant can be used to measure the angle between any two objects, such as between two lighthouses, which will also allow for calculation of a line of position on a chart What is a Sextant? The scale of a sextant has a length of 1⁄6th of a full circle (60°); hence the sextant's name (sextāns, -antis is the Latin word for "one sixth") An octant is a similar device with a shorter scale (1⁄8 of a circle, or 45°), A quintant (1⁄5, or 72°) A quadrant (1⁄4, or 90°) An Octant A Quintant How to handle a Sextant • Please remember, especially for MCA ‘Orals’ examinations that the Sextant is a delicate instrument and must be handled in a particular manner • MCA examiners will be looking for this if they ask you questions relating to the Sextant Sextant Errors Index Error (continued):Correcting using the Star method This is what should be seen when no index error exists If it does exist, this is what will be seen Sextant Errors Index Error (continued):Correcting using the Horizon method Set the sextant to zero Hold the sextant vertically and view a well defined section of the horizon through the eyepiece The true and reflected images should form a straight line Sextant Errors Index Error (continued):Correcting using the Horizon method This is what should be seen when no index error exists If it does exist, this is what will be seen Sextant Errors Index Error (continued):Correcting using the Sun method Set the sextant to zero, hold the sextant vertically and, using the appropriate shades view the sun If there is no error the true & reflected images of the sun will be superimposed on each other like this Sextant Errors Index Error (continued):Correcting using the Sun method Nonetheless, to confirm the presence or not of index error the following procedure is carried out:With the sextant set to zero rotate the micrometer clockwise until the true & reflected images are just touching and take a note of the reading “on” the arc Sextant Errors Index Error (continued):Correcting using the Sun method Then, reverse the images and take a note of the reading “off” the arc Halve the difference between the readings and name the correction “on” or “off” the arc according to the largest reading Reading “on” the arc Reading “off” the arc Difference Half difference = 32.6’ = 31.4’ = 01.2’ = 00.6’ Therefore I.E = 0.6’ and is named “ON THE ARC” (as it is the greater) If the error is “On The Arc” then the error is subtracted If the error is “Off The Arc” then it is added INDEX ERROR N.B Adjusting the 3rd Adjustment screw may well re-introduce side error which will then need to be corrected This may then re-introduce index error Consequently, if Index Error is small it is generally easier to apply the error rather than trying to correct it Errors which cannot be corrected by the Observer Centring Error Worm & Rack Error Optical Errors Graduation Error Collimation Error [...]... furthest from the plane of the instrument Sextant Errors Index Error The horizon mirror and the index mirror must be parallel when the sextant reads 000 00.0’ There are three methods by which we can find out if this error exists :Star method Horizon method Sun method Sextant Errors Index Error (continued):Using the Star method Set the sextant to zero Hold the sextant vertically and view a well defined... Set the sextant to zero Hold the sextant vertically and view a well defined star through the eyepiece The true and reflected images should be in the same vertical plane Side Error (continued):Using the Star method This is what should be seen when no side error exists If it does exist, this is what will be seen Side Error (continued):Using the Horizon method Set the sextant to zero Hold the sextant. .. eyepiece The true and reflected images should be superimposed Sextant Errors Index Error (continued):Correcting using the Star method This is what should be seen when no index error exists If it does exist, this is what will be seen Sextant Errors Index Error (continued):Correcting using the Horizon method Set the sextant to zero Hold the sextant vertically and view a well defined section of the horizon... eyepiece The true and reflected images should form a straight line Sextant Errors Index Error (continued):Correcting using the Horizon method This is what should be seen when no index error exists If it does exist, this is what will be seen Sextant Errors Index Error (continued):Correcting using the Sun method Set the sextant to zero, hold the sextant vertically and, using the appropriate shades view the... the Sextant • When a ray of light is reflected twice in the same plane by two mirrors…… • the angle between the first and last directions of the ray • is twice the angle between the mirrors A Sextant at Work First Error = Perpendicularity:The index mirror must be perpendicular to the plane of the instrument To find out if this error is present , set the index bar near the middle of the arc Hold the sextant. .. superimposed on each other like this Sextant Errors Index Error (continued):Correcting using the Sun method Nonetheless, to confirm the presence or not of index error the following procedure is carried out:With the sextant set to zero rotate the micrometer clockwise until the true & reflected images are just touching and take a note of the reading “on” the arc Sextant Errors Index Error (continued):Correcting ...The parts of a Sextant What is a Sextant? A sextant is a measuring instrument generally used to measure the angle of elevation... calculation of a line of position on a chart What is a Sextant? The scale of a sextant has a length of 1⁄6th of a full circle (60°); hence the sextant' s name (sextāns, -antis is the Latin word for... error exists :Star method Horizon method Sun method Sextant Errors Index Error (continued):Using the Star method Set the sextant to zero Hold the sextant vertically and view a well defined star through