Terrestrial and ship's magnetism :geographical variation and its effect on deviation;

Of course. This is a fundamental topic in marine navigation. Let’s break down terrestrial and ship’s magnetism, how the Earth’s field varies geographically, and how this directly affects a ship’s magnetic compass deviation.

Part 1: Terrestrial Magnetism (The Earth’s Magnetic Field)

The Earth acts like a giant, slightly irregular magnet. This magnetic field, known as terrestrial magnetism, is believed to be generated by the motion of molten iron in the Earth’s outer core. For a navigator, this field is both essential and problematic.

Key Concepts of Terrestrial Magnetism

1. Magnetic Poles vs. Geographic Poles:

   • Geographic Poles (True North/South): These are the points where the Earth’s axis of rotation meets the surface. All maps and charts are oriented to True North.
   • Magnetic Poles (Magnetic North/South): These are the points where the Earth’s magnetic field lines are vertical (pointing straight down or up). They do not align with the geographic poles and, importantly, they wander over time.

2. Geographical Variation (or Magnetic Declination) This is the core of your first question. Variation is the angle between the direction of True North and the direction of Magnetic North at any given location on Earth.

   • How it Varies Geographically: Because the magnetic and geographic poles are in different locations, the angle between them changes depending on where you are on the globe.
     • If you are on the line connecting the geographic and magnetic poles (an agonic line), the variation is zero.
     • In the North Atlantic, for example, Magnetic North is significantly to the west of True North, resulting in a large westerly variation.
     • In parts of Asia, Magnetic North can be to the east of True North, resulting in an easterly variation.
   • Representation: Variation is labelled as East (E) or West (W). This value is printed on all nautical charts, often in the center of a “compass rose,” along with its annual rate of change.

3. Magnetic Dip (or Inclination) This is another crucial aspect of geographical variation. Dip is the angle that the magnetic field lines make with the horizontal plane.

   • Geographical Variation of Dip:
     • At the Magnetic Equator, the field lines are parallel to the Earth’s surface, so the dip is 0°.
     • At the Magnetic Poles, the field lines are perpendicular to the surface (pointing straight down in the north, straight up in the south), so the dip is 90°.
   • Why Dip Matters: Dip is critical because it influences how magnetism is induced in a ship’s steel structure, which directly affects deviation.

Part 2: Ship’s Magnetism

A steel ship is a large mass of magnetic material moving through the Earth’s magnetic field. This causes the ship itself to become a magnet, creating its own magnetic field that interferes with the ship’s compass.

Ship’s magnetism is divided into two types:

1. Permanent Magnetism:

   • Cause: Acquired during construction. The hammering, riveting, and welding of the ship’s steel plates in the Earth’s magnetic field aligns the magnetic domains in the ship’s “hard iron.” This magnetism is relatively stable and fixed in relation to the ship’s structure.
   • Effect: It creates a magnetic field that is constant in strength and direction relative to the ship. As the ship turns, this fixed field affects the compass needle differently on different headings. This causes an error called Semicircular Deviation.

2. Induced Magnetism:

   • Cause: Acquired temporarily from the Earth’s current magnetic field. The ship’s “soft iron” (less magnetically retentive steel) becomes magnetized by induction.
   • Characteristics: This magnetism is not permanent. Its strength and direction depend entirely on two things:
     1. The ship’s heading relative to the Earth’s magnetic field.
     2. The strength and direction (including dip) of the Earth’s magnetic field at its current location.
   • Effect: This causes Quadrantal Deviation (from horizontal soft iron) and Heeling Error (from vertical soft iron when the ship rolls or heels).

Part 3: The Effect of Geographical Variation on Deviation

This is where everything comes together. Deviation is the angle between the direction of Magnetic North and the direction the ship’s compass needle is actually pointing (Compass North). It is the error caused by the ship’s own magnetic field.

Deviation = Magnetic North - Compass North

The crucial link is this: Geographical variation of the Earth’s field directly changes the induced magnetism in the ship, which in turn changes the deviation.

Here’s how:

1. Effect of Changing Latitude (Changing Magnetic Dip):

   • As a ship travels from the equator towards a pole, the magnetic dip increases from 0° towards 90°.
   • The vertical component of the Earth’s magnetic field becomes much stronger.
   • This stronger vertical field induces much stronger magnetism in the ship’s vertical soft iron (masts, funnels, bulkheads).
   • This change in induced magnetism alters the ship’s magnetic signature and therefore changes the deviation values on the compass. It particularly affects the Heeling Error. A ship whose compass was perfectly compensated at the equator will show significant errors as it moves into high latitudes.

2. Effect of Changing Location (Changing Horizontal Field Strength):

   • The horizontal strength of the Earth’s magnetic field also varies across the globe. It is strongest near the magnetic equator and weakest near the poles.
   • The magnetism induced in the ship’s horizontal soft iron (beams, decks) is directly proportional to the strength of the horizontal component of the Earth’s field.
   • As a ship moves to an area with a different horizontal field strength, the magnetism induced in this iron changes, altering the Quadrantal Deviation.

In summary, the permanent magnetism of a ship is (relatively) fixed. However, the induced magnetism is dynamic and changes with every change in the ship’s geographical location. Because the total deviation is a combination of the effects of both permanent and induced magnetism, the deviation itself is not constant. It changes with the ship’s location.

Practical Application and Correction

Navigators cannot ignore these errors. The relationship is as follows:

• True Course: The intended course over the ground, taken from a chart.
• Variation: Applied to get the Magnetic Course.
• Deviation: Applied to get the Compass Course to be steered.

The mnemonic True Virgins Make Dull Company helps remember the order of correction: True -> Variation -> Magnetic -> Deviation -> Compass (When going from True to Compass, you Add West / Subtract East variations/deviations).

The reverse is used to convert a compass bearing to a true bearing: Can Dead Men Vote Twice (at elections) Compass -> Deviation -> Magnetic -> Variation -> True (When going from Compass to True, you Add East / Subtract West).

Because deviation changes with location, a ship’s Deviation Card (a table of deviation values for different headings) is only accurate for the specific magnetic latitude where the compass was last “swung” (calibrated). For long north-south voyages, the compass must be checked and potentially re-compensated to account for the changes in deviation caused by the geographical variation of the Earth’s magnetic field.