The angular distance of a celestial body between it's position in space and a point in space directly above a known position on earth is called the 'zenith distance'. It is used to calculate the true bearing of the body, the Azimuth, and also the angular height, or altitude, of the body above the celestial horizon from that position. Sometimes this is called a 'trial position' but we know it as an estimated position. At the same time the altitude of the body from the horizon is measured at the observer's unknown position using a sextant. It is then corrected to make it look as though it was taken from the centre of the earth in order to use the celestial horizon to find the true altitude of the body. Subtracting the calculated altitude from the true altitude gives a short distance called the 'intercept' from the estimated position to the observer's position which can easily be plotted on a chart on the bearing of the Azimuth to give a position line. Repeating the process for another body at about the same time produces another position line and where they cross is a fix. This book seeks to describe the theory behind the Intercept method of sight reduction and to guide the navigator through the practice of taking and plotting the sight.