13. Terrestrial and ship's magnetism :

Excellent topic. This is a fundamental and critical area of knowledge for any mariner, as it directly relates to the use and accuracy of the magnetic compass for navigation.

Here is a comprehensive breakdown of Terrestrial and Ship’s Magnetism.

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Overview

For safe navigation, a mariner needs to know their direction relative to True North (the direction of the geographic North Pole). A magnetic compass, however, does not point to True North. It is influenced by two primary magnetic fields:

1. Terrestrial Magnetism: The Earth’s own magnetic field.
2. Ship’s Magnetism: The magnetic field created by the ship itself.

Understanding and correcting for the effects of both is essential for accurate navigation.

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

The Earth behaves like a giant, imperfect magnet.

1. The Source

The Earth’s magnetic field is believed to be generated by the dynamo effect in its molten outer core. The movement of liquid iron, driven by convection and the Earth’s rotation, creates electrical currents, which in turn produce the magnetic field.

2. Key Characteristics

• Magnetic Poles: The Earth has a North Magnetic Pole and a South Magnetic Pole. Crucially, these are not in the same location as the geographic poles. Furthermore, they wander over time.
• Magnetic Field Lines: Lines of magnetic force flow from the South Magnetic Pole, around the Earth’s surface, and into the North Magnetic Pole. A compass needle aligns itself with these lines of force.
• Horizontal and Vertical Components: At any point on Earth, the magnetic field can be broken down into two components:
  • Horizontal Force (H): This is the component that directs the compass needle to point north-south. It is strongest at the magnetic equator and weakest near the magnetic poles.
  • Vertical Force (Z) / Magnetic Dip: This is the component that pulls the compass needle downwards (in the Northern Hemisphere) or upwards (in the Southern Hemisphere). This angle of “dip” is 0° at the magnetic equator and 90° at the magnetic poles. This force doesn’t help with direction but can cause the compass card to tilt.

3. The Navigational Error: Variation (or Declination)

Because the magnetic and geographic poles are in different locations, a magnetic compass will not point to True North.

Variation is the angle between the direction of True North and the direction of Magnetic North at a specific location on Earth.

• It is named East or West depending on whether Magnetic North is to the east or west of True North.
• Variation changes with geographic location and also changes slowly over time (secular change).
• The value of variation for any given area is printed on nautical charts.