Terrestrial and ship's magnetism :variation, dip, total force, horizontal force, vertical force, local attraction;

Of course. This is a fundamental topic in marine navigation. Let’s break down each of these concepts, starting with the Earth’s magnetism and then moving to how a ship interacts with it.

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

The Earth behaves like a giant, slightly irregular magnet. This is thought to be caused by the movement of molten iron in its outer core, creating a “dynamo effect.” This magnetic field is crucial for navigation using a magnetic compass.

1. Variation (or Magnetic Declination)

• Definition: Variation is the angle at any given location between True North (the direction to the geographic North Pole) and Magnetic North (the direction the north-seeking end of a compass needle points, influenced by the Earth’s magnetic field).
• Why it exists: The Earth’s magnetic poles are not in the same location as its geographic poles.
• How it’s measured: In degrees East (E) or West (W).
  • Variation East: Magnetic North is to the east of True North.
  • Variation West: Magnetic North is to the west of True North.
• Importance: A navigator must apply variation to a magnetic compass bearing to find the true direction on a nautical chart. This value is printed on the compass rose of every chart, along with its annual rate of change.

2. Dip (or Magnetic Inclination)

• Definition: Dip is the angle that a freely suspended magnetic needle, able to pivot vertically, makes with the horizontal plane.
• Why it exists: The Earth’s magnetic lines of force are only parallel to the surface at the magnetic equator. As you move towards the magnetic poles, they curve downwards (in the Northern Hemisphere) or upwards (in the Southern Hemisphere).
• How it’s measured:
  • At the Magnetic Equator, Dip is 0°. The needle is perfectly horizontal.
  • At the Magnetic Poles, Dip is 90°. The needle points straight down (North) or straight up (South).
• Importance: Dip causes the vertical force component of magnetism. It can make a compass card sluggish or stick, so compasses must be balanced for the hemisphere they are intended for.

3. The Components of Magnetic Force

The Earth’s magnetic field at any point is a vector quantity, meaning it has both strength and direction. This is called the Total Force (F). We can break this force down into two key components using simple trigonometry, with Dip as the angle.