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* The reason that the chosen AP is not important (within limits) is that if a position closer to the body is chosen then Hc will be greater but the distance will be measured from the new AP which is closer to the body and the end resulting LOP will be the same.
▲Suitable bodies for celestial sights are selected, often using a Rude Star Finder. Using a [[sextant]], an altitude is obtained of the sun, the moon, a star or a planet. The name of the body and the precise time of the sight in [[UTC]] is recorded. Then the sextant is read and the altitude (Hs) of the body is recorded. Once all sights are taken and recorded, the navigator is ready to start the process of "sight reduction" and plotting.
The first step in sight reduction is to correct the sextant altitude for various errors and corrections. The instrument may have an error, IC or index correction (See article on [[sextant#Adjustment|adjusting a sextant]]). Refraction by the atmosphere is corrected for with the aid of a table or calculation and the observer's height of eye above sea level results in a "dip" correction, (as the observer's eye is raised the horizon dips below the horizontal). If the Sun or Moon was observed, a semidiameter correction is also applied to find the centre of the object. The resulting value is "observed altitude" (Ho).
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