Intercept method: Difference between revisions

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== Summary ==

The intercept method is based on the following principle.

The actual distance from the observer to the geographical position ('''GP''') of a celestial body (that is, the point where it is directly overhead) is "measured" using a [[sextant]]. The observer has already estimated his position by [[dead reckoning]] and calculated the distance from the estimated position to the body's GP; the difference between the "measured" and calculated distances is called the intercept.

[[Image:Diagram showing GP distance = ZD.jpg|thumb|right|500px]]

The diagram on the right shows why the zenith distance of a celestial body is equal to the angular distance of its GP from the observer's position.

The rays of light from a celestial body are assumed to be parallel (unless the observer is looking at the moon, which is too close for such a simplification). The angle at the centre of the earthEarth that the ray of light passing through the body's GP makes with the line running from the observer's [[zenith]] is the same as the zenith distance. This is because they are [[corresponding angles]]. In practice it is not necessary to use zenith distances, which are 90° minus altitude, as the calculations can be done using observed altitude and calculated altitude.

Taking a sight using the intercept method consists of the following process: