Accurately speaking, the Bearing of a Heavenly Body, say the Sun, differs at every point of a circle of equal Altitudes but if the Altitude of the Sun is not very near the size of the circle is so great that the Sun's Bearing does not alter appreciably over a segment of the circle of say 40 or 50 miles, and this segment of the circle may, for practical purposes, be considered a straight line. The Azimuth of the Sun is therefore at right angles to that straight line, and the straight line, which is of course the 'line of position' of the ship, is at right angles to the Azimuth.