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Sextant Navigation - The Altitudes
In marine navigation, when a navigator measures the altitude of a celestial body with a marine sextant he has to measure the altitude as an angle of the body above the visible horizon. (Click on the picture to enlargen.)
The altitude thus obtained has to be corrected for instrument and other errors before calculations can be made.
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The Marine Sextant
The sextant derives its name from the extent of its limb which is the sixth part of a circle, or 60 degrees. The marine sextant is a double reflection instrument, used for measuring angles in then same plane. The arc is graduated into degrees from right to left from 0 to 120 (sometimes a little more). However the limb is only 1/6th of a circle due to the instrument double reflecting. See the adjoining picture for more clarification.
To the right of 0 degrees on the arc is graduated 5 degrees. This arc of excess is called ‘Off the arc’, and the arc to the left of the zero is termed ‘On the arc’.
The sextant can be used to measure angles in vertical, horizontal or oblique planes.
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The Principle of the Sextant
The optical principle used in a sextant is this: given that 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.
This is the angle that is read off on the ‘arc’ after the navigator ‘shoots a star’ (takes an altitude).
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Errors and Corrections to Sextant Readings
Besides errors during manufacture, which is rare, a marine sextant can have errors due to:
- the index mirror not being perpendicular to the plane of the instrument.
- the horizon glass not been perpendicular to the plane of the instrument
- the horizon and the index mirrors not being perpendicular to each other
At sea, these errors are corrected by adjusting screws located on the index and horizon mirrors. Any residual error is called index error and is calibrated by the observer.
Other corrections to sextant readings include Dip (for height of eye of the observer), refraction (for refraction of light in the atmosphere, parallax (we are measuring altitude at the surface of the earth and not at the center of the earth, which is true altitude), semi diameter (of bodies like the sun and other planets. This is because the true altitude of the body is from it’s center whereas the navigator cannot know that exactly, and so uses the outer circumference of the sun or planet).
All these corrections are calculated or found in nautical Almanacs and tables and applied.
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How is a Sight Taken?
A navigator will decide which stars or planets to 'shoot', depending on time of day, clarity of the horizon and the heavenly bodies (stars and planets) that he or she can see clearly.
The taking of the altitude involves 'bringing the celestial body down to the horizon' when seen through the sextant. This means that the reflection of the body, as seen through the sextant, is slowly lowered down to a point when it is just touching the horizon. This is done by first setting the sextant to zero and slowly and smoothly moving the arm of the sextant, all the while keeping the reflection of the body in sight within the instrument. Finer adjustments are made with the micrometer.
When the navigator is satisfied that the body is perfectly on the horizon (with planets and the sun, the sextant is rocked from side to side to ensure that the circumference of the body is tangential to the horizon), the angle is then read off the scale on the sextant. This is the uncorrected altitude.
A navigator would typically go out on the navigation bridge wing. He or she then takes an altitude and notes the exact time. Calculations are then made to determine the ship’s position. This is usually done when the ship is far from land, and lighthouses and other land based objects cannot be used to ascertain position.
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With the advent of the GPS, the marine sextant is slowly becoming obsolete. However, marine navigational examinations still test on the proper use of a sextant. Electronic devices like the GPS are considered aides to navigation. The sextant is the real thing.