Editing Navigation (section)

====The marine sextant====
[[File:Marine sextant.svg|thumb|upright=1.2|The marine [[sextant]] is used to measure the elevation of celestial bodies above the horizon.]]
{{Further|Sextant}}
The second critical component of celestial navigation is to measure the angle formed at the observer's eye between the celestial body and the sensible horizon. The sextant, an optical instrument, is used to perform this function. The sextant consists of two primary assemblies. The frame is a rigid triangular structure with a pivot at the top and a graduated segment of a circle, referred to as the "arc", at the bottom. The second component is the index arm, which is attached to the pivot at the top of the frame. At the bottom is an endless vernier which clamps into teeth on the bottom of the "arc". The optical system consists of two mirrors and, generally, a low power telescope. One mirror, referred to as the "index mirror" is fixed to the top of the index arm, over the pivot. As the index arm is moved, this mirror rotates, and the graduated scale on the arc indicates the measured angle ("altitude").<ref name="q939">{{cite book | last=House | first=D.J. | title=Seamanship Techniques | publisher=Routledge | publication-place=London | date=2013-11-12 | isbn=978-1-135-08015-0 | page=350-252}}</ref> The second mirror, referred to as the "horizon glass", is fixed to the front of the frame. One half of the horizon glass is silvered and the other half is clear. Light from the celestial body strikes the index mirror and is reflected to the silvered portion of the horizon glass, then back to the observer's eye through the telescope. The observer manipulates the index arm so the reflected image of the body in the horizon glass is just resting on the visual horizon, seen through the clear side of the horizon glass.<ref name="q939"/>

There are three main errors that must be corrected in order to each usage for navigation.<ref name="q939"/> The main errors are perpendicular error, side error and index error.<ref name="q939"/> Adjustment of the sextant consists of checking and aligning all the optical elements to eliminate the overall "index error" (or index correction). Index correction should be checked, using the horizon or more preferably a star, each time the sextant is used.<ref name="q939"/> The practice of taking celestial observations from the deck of a rolling ship, often through cloud cover and with a hazy horizon, is by far the most challenging part of celestial navigation.<ref>{{Cite web |title=How Did Aviators "Shoot" the Sun and Stars? {{!}} Time and Navigation |url=http://timeandnavigation.si.edu/navigating-air/challenges/overcoming-challenges/celestial-navigation |access-date=2023-06-12 |website=timeandnavigation.si.edu |language=en}}</ref>