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* Observe the altitude above the horizon '''Ho''' of a celestial body and note the time of the observation.
* Assume a certain geographical position (lat., lon.), it does not matter which one so long as it is within, say, 50 NM of the actual position (or even 100 NM would not introduce too much error). Compute the altitude '''Hc''' and azimuth '''Zn''' with which an observer situated at that assumed position would observe the body.
* If the actual observed altitude Ho is smaller than the computed altitude Hc this means the observer is farther away from the body than the observer at the assumed position, and viceversa. For each minute of arc the distance is one NM and the difference between Hc and Ho expressed in minutes of arc (which equal NM) is termed the "intercept". The navigator now has computed the intercept and azimuth of the body.
* On the chart he marks the assumed position '''AP''' and draws a line in the direction of the azimuth Zn. He then measures the intercept distance along this azimuth line, towards the body if Ho>Hc and away from it if Ho<Hc. At this new point he draws a perpendicular to the azimuth line and that is the line of position '''LOP''' at the moment of the observation.
* 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.
==Methodology==
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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.
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Any sight can be advanced and used to obtain a ''running fix''. It may be that the navigator due to weather conditions could only obtain a single sight at dawn. The resulting LOP can then be advanced when, later in the morning, a Sun observation becomes possible. The precision of a running fix depends on the error in distance and course so, naturally, a running fix tends to be less precise than an unqualified fix and the navigator must take into account his confidence in the exactitude of distance and course to estimate the resulting error in the running fix.
Determining a fix by crossing LOPs and advancing LOPs to get running fixes are not specific to the intercept method and can be used with any sight reduction method or with LOPs obtained by any other method (bearings, etc.).
== See also ==
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