See also: Sextant (homonymy)

A sextant is a Instrument of navigation making it possible to raise the angular Height of a star above the horizon. It is used to show the not out of sight of ground (the article: astronomical Navigation). The sextant is always used in the Aéronautique, the marine , the terrestrial raids, etc although its use is restricted because of the development of the systems of positioning by satellites.

History

The Greek ancient and Byzantine S already used for the Navigation Astrolabe S and octant S, such those found in Anticythère in a wreck of the 3rd century before our era; Héron of Alexandria (Ier century) makes description of it.

However, the modern sextant was invented in the Années 1730 by two people independently one of the other: John Hadley (1682 - 1744), a Mathematician English, and Thomas Godfrey (1704 - 1749), an American inventor .

The specificity of the sextant compared to the astrolabe is that the two directions which one wants to measure the angle are observed at the same time , making measurement about independent of the movements of the ship. The sextant is held with height of the eyes, whereas the astrolabe requires a all the more high point of suspension as one aims at a star of high site.

Precision of measurements and adjustment

The reading of a well regulated sextant allows an accuracy of 0,2 ' of arc. In theory, an observer could thus determine its position with an accuracy of 0,2 nautical miles (since 1 thousand corresponds to 1 ' of arc of large circle), that is to say approximately 350 meters. In practice, the navigators obtain a precision of about 1 or 2 marine miles (movements of the ship, swells, horizon more or less Net, inaccuracies of the hour or the estimates between the successive aimings of the same star or different stars).

The instrumental errors of the sextant are the eccentricity and collimation.

• the eccentricity is a clean data of the sextant to construction and cannot be corrected. It is function measured height and is registered in the box of the sextant;
• collimation can be regulated and it is necessary to check it before each observation by superimposing the direct image of a star and its considered image and conversely, collimation being equal to the average of these two measurements.

If collimation exceeds 3 ', it is necessary to check and rectify:

• the optical axis (old sextants), which must be parallel to the plan of the limb, by comparing the aimings of a test card with 30 Mr. by the glasses and riders posed on the limb. One then operates the adjustable tangents of the collar carry-glasses;
• the large mirror, which must be, by comparing the direct aiming of a rider with the aiming reflected by the large mirror of a second rider. One then operates the adjustable tangent of the large mirror;
• the small mirror, which must be perpendicular to the plan of the limb and parallel to the large mirror, by aiming at a distant point or the horizon: the two images must be confused and remain it by inclining the sextant. One operates the adjustable tangents of the small mirror.

Levelling of a star to the sextant

The observation consists in “making descend” the image considered from the star on the horizon and do it tangenter the horizon (from where the movement of beam of the hand which holds the sextant). If it is of the sun or the moon, one makes tangenter his edge lower or higher. For stars and planets, it is advised “to assemble the horizon” in the vicinity of the star while turning over the sextant, then to observe normally.

The height measured with the sextant must be corrected instrumental errors and of a certain number of parameters suitable for the height of the observer above water, to the astronomical refraction and the star concerned.

The true height $h\_v\; \backslash ,$ is deduced the measured height $h\_m\; \backslash ,$par the formula:

$h\_v\; =\; h\_m\; +\; \backslash \; epsilon\; +\; C\; -\; D\; -\; R\; +\; P\; \backslash \; pm\; \backslash \; delta\; \backslash ,$

with:

$\backslash \; epsilon\; \backslash ,$, the eccentricity of the sextant;

$c\; \backslash ,$, the collimation of the sextant;

$d\; \backslash ,$, depression of the horizon, function eye level of the observer, given by the éphémérides;

$R\; \backslash ,$, the astronomical refraction;

$P\; \backslash ,$, the parallax (negligible for stars and planets);

$\backslash \; delta\; \backslash ,$, the semi-diameter (apparent) of the moon or the sun, affected of the sign + if the lower edge were aimed at, of the sign - if the higher edge were aimed at.

For the sun, the éphémérides give the value day laborer of $\backslash \; delta\; \backslash ,$ as well as the sum $-\; D\; -\; R\; +\; P\; +\; \backslash \; delta\_m\; \backslash ,$; $\backslash \; delta\_m\; \backslash ,$ being the average semi-diameter and one applies a second correction: $+\; (\backslash \; delta\; -\; \backslash \; delta\_m)\; \backslash ,$ for the lower edge and $-\; (\backslash \; delta\; +\; \backslash \; delta\_m)\; \backslash ,$ for the higher edge.

For the moon one applies a similar formula with values given by the éphémérides.

For stars and planets: $\backslash \; delta\; \backslash ,$ is negligible; $P\; \backslash ,$ is negligible, except for Mars and Venus. The sum $-\; (D\; +\; R)\; \backslash ,$ is provided by the éphémérides as well as the value of $P\; \backslash ,$ for Mars and Venus.

Other uses

Distance from a Amer

One measures with the sextant the angular height of a land-mark which one knows the height. It is advisable however to be careful:

• the building must be completely visible: it should not have the feet in water and not be partly behind the horizon;
• one should not confuse the height of the building with the rise in the hearth of a headlight or a fire, which only is mentioned on the sea charts and is counted since the level of the open seas of spring tides (coefficient 95).

The distance $D\; \backslash ,$ into nautical: $D\; =\; 1,852\; \backslash \; frac\; \{H\}\; \{h\_i\}\; \backslash ,$ with $H\; \backslash ,$ the height of the building in Mr. and $h\_i\; \backslash ,$ the instrumental height in minutes.

Horizontal angles

By using the sextant in the horizontal plane, it is possible to measure the angle between two objects. This method makes it possible to make a point by able arcs; to see the article: Coastal navigation.

See too

Related articles

• Angle
• astronomical Astrolabe
• Navigation
• Octant
• Quadrant
• Sphere armillaire

External bonds

• See a detailed diagram of the Sextant.

• See a general diagram of the Sextant animated by a Cabrijava applet.
• See examples of Sextants.

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