The intercept method is based on the following principle.
The actual distance from the observer ofto the geographical position ('''GP''') of a celestial body (that is, the point where it is directly overhead) is "measured" using a sextant,. thisThe distanceobserver ishas equalalready toestimated itshis zenithposition distance.by The[[dead distancereckoning]] is thenand calculated the distance from where the observerestimated believes himselfposition to bethe throughbody's [[deadGP; reckoning]]. Thethe difference between the two"measured" is thenand calculated by subtracting one from the other. This distancedistances 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.
