old navy

SEXTANT

When a ship is approaching a rocky coast and the life of the ship and its crew depends on a fast and accurate answer. It's the Navigator's job to provide the answer. So what do navigators need to find their position on the earth's surface by observing the stars? They need an Almanac prepared by the astronomers to forecast precisely where the heavenly bodies, the sun, moon planets and selected navigational stars, are going to be, hour by hour, years into the future, relative to the observatory that prepared the almanac, Greenwich, England in modern times. They need a chronometer or some other means of telling the time back at the observatory that was the reference point for the data in the almanac, It is the cartographer's job to provide accurate charts so that navigators can establish their position in latitude and longitude or in reference to landmasses or the hazards of rocks and shoals. The navigators need a quick and easy mathematical method for reducing the data from their celestial observations to a position on the chart Finally, navigators need an angle-measuring instrument, a sextant, to measure the angle of the celestial body above a horizontal line of reference. How do navigators use the stars, including our sun, the moon, and planets to find their way? Well, for at least two millennia, navigators have known how to determine their latitude, their position north or south of the equator. At the North Pole, which is 90 degrees latitude, Polaris (the North Star) is directly overhead at an altitude of 90 degrees. At the equator, which is zero degrees latitude, Polaris is on the horizon with zero degrees altitude. Between the equator and the North Pole, the angle of Polaris above the horizon is a direct measure of terrestrial latitude. If we were to go outside tonight and look in the northern sky, we would find Polaris at about 40 degrees 13 minutes altitude - the latitude of Coimbra. In ancient times, the navigator who was planning to sail out of sight of land would simply measure the altitude of Polaris as he left homeport, in today's terms measuring the latitude of home port. To return after a long voyage, he needed only to sail north or south, as appropriate, to bring Polaris to the altitude of home port, then turn left or right as as appropriate and "sail down the latitude," keeping Polaris at a constant angle. The Arabs knew all about this technique. In early days, they used one or two fingers width, a thumb and little finger on an outstretched arm or an arrow held at arms length to sight the horizon at the lower end and Polaris at the upper.

he sextant basically consists of a telescope, a half silvered horizontal mirror which the telescope "looks" through and a moving arm on which the index mirror is fixed. By manipulating this arm a star or other celestial body can be made to appear on the horizon. Accurate adjustments are made by means of a micrometer knob. The angle can then be read off the arc and micrometer. The shades are to use when the object being looked at is bright - such as the sun.

he sextant relies on the optical principle that if a ray of light is reflected from two mirrors in succession then the angle between the first and last direction of the ray is twice the angle between the mirrors. And this angle can then be read off the arc.To use the sextant the telescope must be focused on the horizon. The celestial body to be shot, found and the sextant aimed at it. Bring the body down to the horizon by moving the arm along the arc and then clamp the arm. Using the micrometer knob make small adjustments while gently swaying the instrument slightly from side to side until the heavenly body just brushes the horizon.  When this is achieved instantly make a note of the time, seconds first, then minutes and hours, then the name of the body and its observed altitude. Every second of time counts - an error of 4 seconds equates to an error of a nautical mile in the position.