Navigation

NAU 102

Lesson 14

Magnetism & Compasses

A basic function of navigation is finding direction.

We must determine:

CoursesHeadingsBearings

Magnetism & Compasses

Primary instruments for determining direction:

Magnetic Compasses

Gyrocompasses

Magnetism

The physical phenomenon by which certain metals attract or

repel items of like material.

One way to find direction on Earth is to use the phenomenon of

magnetism.

Magnet

An object that exhibits the property of magnetism.

Can occur naturally (a lodestone) or be induced in certain metals by:•Contact with a lodestone

•Exposure to an electric or magnetic field

Lodestone

Magnet Field

Magnets produce a force that can affect the surrounding area.

Magnet Field

The effect of the magnetic field can be illustrated by lines of force.

Magnet Poles

Every magnet has two poles. The lines of force flow from one (called the “north pole”) to the other (the “south pole”).

Magnet Poles

Alike poles repel each other.

Magnet Poles

Opposite poles attract each other.

Our Magnetic Earth

The Earth has a magnetic field.

This can be visualized by imagining a bar magnet inside the Earth.

Our Magnetic Earth

Magnetic Poles

The North & South magnetic poles are not in the same locations as the geographic

poles.

Magnetic North ≈ 78.9°N 103.8°W

Magnetic South ≈ 64.5°S 139.5°E

The location of the poles are constantly changing.

Dip

The magnetic lines of force have vertical and horizontal components.

Dip – the vertical angle of lines of force.

0° at the magnetic equator

90° at the poles

Dip

A Little Review

True North (0° T) – the direction from you to the geographic north pole; it is measured

along your meridian.

Meridian – the great circle passing through the geographic poles and a position on the

Earth.

Magnetic Directions

Magnetic North (0° M) – the direction of the magnetic meridian that passes through your

position.

Magnetic Meridian – a line of force running between the magnetic poles, passing

through a position on the Earth.

Magnetic Directions

Why Not?

Magnetic Directions ≠ True Directions

Geographic North Pole is in a different place than the Magnetic North Pole.

Magnetic meridians aren’t great circles.

Variation

“The difference at a location between the directions of the magnetic and true

meridians.”“The difference between true north and the direction the compass would point if free of

all local disturbances.”

- Dutton’s

Variation

Local variation is shown on the compass rose nearest your

location.

It changes over time.

Variation

Labeled West (W) if 0°M is west (or to the left) of 0°T.

Labeled East (E) if 0°M is east (or to the right) of 0°T.

Introduction to Navigation

Questions?