A celestial position line is a line as a part of the circle drawn on the earth’s surface where the centre is the geographical position (GP) of the celestial body, with a radius equal to the Zenith Distance. The position line would be perpendicular to the bearing (Azimuth) of the celestial body from the observer. Let’s suppose the ship is at O, and the celestial body X bears 254° T. The circle is drawn with radius XO, and centred at the geographical position of body X is the position circle. The arc at O that is a part of the position circle is the position line, and, of course, is perpendicular to XO, which is the bearing of the body X from the ship. This position line is just a short line, and is considered a straight line tangent to the circle at O, so this particular position line is running 164°/344°. When a position line is found, the ship’s position is somewhere on it. If another position line is found, the intersection of the two position lines is the ship’s position. As shown in the figure below, two intersections are made by two position circles; the bearing of the body will identify which is the corrected one.
