CHAPTER V. SHIP'S HEADING 



43. Ship's heading. — Ship^s heading is the angle, expressed in de- 

 grees clockwise from north, of the ship's fore-and-aft line with re- 

 spect to the true meridian or the magnetic meridian. When this 

 angle is referred to the tme meridian^ it is called a triie heading. 

 When this angle is referred to the magnetic meridian.^ it is called a 

 magnetic heading. Heading, as indicated on a particular compass, 

 IS termed the ship's compass heading by that compass. It is always 

 essential to specify heading as true heading, magnetic heading, or 

 compass heading. In order to obtain the heading of a ship, it is 

 essential that the line through the pivot and the forward lubber's 

 line of the compass be parallel to the fore-and-aft line of the ship. 

 This applies also to the peloruses and gyro repeaters, which are used 

 for observational purposes. 



44. Variation. — Variation is the angle between the magnetic me- 

 ridian and the true meridian, measured from true north. If this 

 angle is to the right of the true meridian.^ the variation is easterly .^ 

 and if the angle is to the left of the true meridian., the variation is 

 westerly. The local variation and its small annual change are noted 

 on the compass rose of all navigational charts. Thus the true and 

 magnetic headings of a ship differ by the local variation. Figure 6 

 presents approximate variation values for the world, adapted from 

 H. O. Chart 2406. 



45. Deviation. — As previously explained, a ship's magnetic influ- 

 ence will generally cause the compass needle to deflect from the 

 magnetic meridian. This angle of deflection is called deviation. If 

 the north end of the needle points east of the inagnetic meridian.^ the 

 deviation is easterly; if it points west of the magnetic meridian, the 

 deviation is westerly. 



46. Heading relationships. — A summary of heading relationships 

 follows : 



(1) Deviation is the difference between the compass heading 

 and the magnetic heading. 



(2) Variation is the difference between the magnetic heading 

 and the true heading. 



(3) The algebraic sum of deviation and variation is the com- 

 pass error. 



35 



