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.
Marine Navigational Equipment: The Sextant
written by: Manu • edited by: Lamar Stonecypher • updated: 5/14/2013
Many of us have seen photographs of seamen using a marine sextant. What exactly is that navigator measuring? Learn how a sextant works to help marine navigators find their position.
Sextant Navigation - The Altitudes
sextant
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.
The altitude thus obtained has to be corrected for instrument and other errors before calculations can be made.
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.
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.
http://www.brighthubengineering.com/seafaring/31615-marine-navigational-equipment-the-sextant/