In celestial navigation, the difference in minutes of arc between the observed altitude and the calculated altitude, used to plot a position line.
In practice
The intercept is the key quantity linking the raw celestial observation to the navigational chart. An OOW who understands the intercept concept can assess whether a sight has been well taken—a very large intercept suggests an error in the assumed position, the almanac extraction, or the sight itself—and can correct errors before plotting. Modern scientific calculators and dedicated celestial navigation programs compute the intercept automatically, but STCW requires officers to understand the method well enough to work manually, ensuring they can navigate by celestial means if electronic aids are unavailable.
Regulatory detail & full definition
In celestial navigation, the intercept (also called the intercept distance) is the difference between the observed altitude (Ho) of a celestial body and the calculated altitude (Hc) computed for an assumed position at the time of observation. It is expressed in minutes of arc, which are equivalent to nautical miles, and indicates how far the true position line lies from the assumed position: toward the body if Ho exceeds Hc, and away from the body if Hc exceeds Ho. The direction to plot this offset is the body's azimuth (Z), derived from the same sight reduction. Bowditch (American Practical Navigator) attributes the intercept method to the French navigator Marq de Saint-Hilaire, and it remains the universal method of celestial position-line plotting.
The intercept is the key quantity linking the raw celestial observation to the navigational chart. An OOW who understands the intercept concept can assess whether a sight has been well taken—a very large intercept suggests an error in the assumed position, the almanac extraction, or the sight itself—and can correct errors before plotting. Modern scientific calculators and dedicated celestial navigation programs compute the intercept automatically, but STCW requires officers to understand the method well enough to work manually, ensuring they can navigate by celestial means if electronic aids are unavailable.