Sight reduction

Sight reduction is the process of deriving from a sight the information needed for establishing a line of position.

Sight is defined as the observation of the altitude, and sometimes also the azimuth, of a celestial body for a line of position; or the data obtained by such observation.

Nowadays sight reduction uses the equation of the circle of equal altitude to calculate the altitude of the celestial body,

${\displaystyle \sin(Hc)=\sin(B)\sin(Dec)+\cos(B)\cos(Dec)\cos(LHA)}$

and the azimuth Zn is obtained from Z by:

${\displaystyle \cos(Z)=(\sin(Dec)-\sin(Hc)\sin(B))/(\cos(Hc)\cos(B))}$

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