Intercept method: Difference between revisions

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The '''"Intercept Method'''", or '''"Marcq St Hilaire method'''", as it is also rather inaccurately known, is an astronomical [[navigation]] method of calculating an observer's position on earth. It was originally called the ''azimuth intercept'' method because the process involves drawing a line which intercepts the azimuth line. This name was shortened to ''intercept'' method and the ''intercept distance'' was shortened to ''"intercept''".

The method yields a line of position (LOP) on which the observer is situated. The intersection of two or more such lines will define the observer's position, called a ''"fix''". Sights may be taken at short intervals, usually during hours of twilight, or they may be taken at an interval of an hour or more (as in observing the Sun during the day). In either case, the lines of position, if taken at different times, must be advanced or retired to correct for the movement of the ship during the interval between observations. If observations are taken at short intervals, a few minutes at most, the corrected lines of position by convention yield a "fix". If the lines of position must be advanced or retired by an hour or more, convention dictates that the result is referred to as a "running fix".

== Summary ==

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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 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.