HEADING 39 



sons, the gyro error must be checked constantly, especially if the gyro 

 IS being used to obtain data for determining residual deviation curves 

 of the magnetic compass. 



49. A ship may be placed on a magnetic heading by aligning the 

 vanes of an azirrmth circle with the sun over the topside compass. The 

 sun is a distant object whose azimuth (angle from the north) may be 

 computed for any given time. Methods of calculating sun's azimuths 

 are discussed in chapter VI. By setting the line of sight of the 

 vanes at an angle to the right (or left) of the fore-and-aft line of the 

 ship equal to the difference between the computed magnetic azimuth 

 and the desired magnetic heading of the ship, and then swinging the 

 ship until the sun is aligned with the vanes, the ship will be on the 

 desired magnetic heading. Simple diagrams (as in fig. 25) with the 

 ship and the sun drawn in their relative positions, will aid greatly in 

 the visualization of each problem. The azimuth circle must always 

 be kept level while making observations, particularly of celestial 

 bodies. 



Ship's Desired 

 Heading 90° 



Ship's 

 Desired Heading 

 225° Magnetic Sun 



Magnetic 115° 



FiGUEH 25. — Azimuth circle set-ups. 



50. A distant object (ten or more miles away) may be used in con- 

 junction with the azimuth circle for placing the ship on magnetic head- 

 ings, provided the ship stays within a small area. This procedure is 

 similar to that used with the sun except that the magnetic bearing of 

 the object is constant. With an object 11.4 nautical miles distant, a 

 change in position of 400 yards at right angles to the line of sight 

 introduces an error of 1°. 



51. A pelorus may be used to place a ship on a magnetic heading 

 using the sun^s azirrvuth in much the same manner as with the azimuth 

 circle. Use of the pelorus has the further advantage in that the 



