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How To Use A Sextant

October 13th, 2010

Brass Sextants

Upon first glance at the sextant one may feel intimidated by its many accessories. And the truth is, you really shouldn’t be. The basic use of the sextant is fairly easy; however, it is the charting, locating, and plotting that is the hardest part of using a sextant properly. The sextant works to find your position relative to a celestial object, such as the sun, moon, or stars. The celestial object is positioned over the horizon by way of two mirrors on the sextant that allow the navigator to determine the latitude they are currently in. The sextant can even work properly while on a moving ship. This is because the sextant sees the horizon directly and as unmoving, while the celestial object is viewed through two opposing mirrors that take into account the motion of the sextant from the reflection.

The sextant’s scale is one sixth that of a complete circle, which gives it the name “sextant”. The sextant’s two mirrors work in conjunction with the telescope, scale, and filters to determine an extremely accurate location. The mirror through which the navigator sees the horizon is half silvered to allow for light to come through. The second mirror opposing the first is attached to a movable arm that glides along the scale. The arm is moved so that the second mirror positions a celestial object’s light into the first mirror to give the appearance that the object is directly on the horizon. The angle between the two superimposed objects can be told by the points on the scale. This is the basic way of how to use a sextant.

Navigating the seas with a sextant is a very tricky affair that involves a lot of recalculating and astronomical references. The way to find latitude is by measuring the angle between sun and horizon while the sun sits at it’s highest point. The sun will be in the proper position at noon. From there you will have to reference a table compiled by astronomers that reveals where the sun should be at that particular time on that particular day. Due to the Earth’s steady motion, navigators can deduce that for every hour the sun will move 15 degrees. To measure this accurately and effectively navigators make use of a chronometer. A chronometer is basically an extremely accurate clock.

Using the sextant for celestial navigation as it was intended is a bit tougher. Celestial navigation measures angles in the sky in relation to the horizon in order to find the navigators global positioning. The position on Earth where the celestial object is aligned is known as the sub point, the location of which is found by referring to tables.

The measured angle of the celestial object and the horizon is directly correlated to the celestial object’s sub point and the navigators position. The measurement of this correlation defines a circle known as the celestial line of position (LOP). Of which the location and size is discovered by way of mathematical equations.

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