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Navigation

NAU 102

Lesson 14

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Magnetism & Compasses

A basic function of navigation is

finding direction.

We must determine:

Courses

Headings

Bearings

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Magnetism & Compasses

Primary instruments for

determining direction:

Magnetic Compasses

Gyrocompasses

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Magnetism

The physical phenomenon by

which certain metals attract orrepel items of like material.

One way to find direction on Earth

is to use the phenomenon of

magnetism.

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Magnet

An object that exhibits the property

of magnetism.

Can occur naturally (a lodestone) orbe induced in certain metals by:

Contact with

a lodestoneExposure to an electric or

magnetic field

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Magnet Field

Magnets produce a force that can affect the

surrounding area.

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Magnet Field

The effect of the magnetic field can be

illustrated by lines of force.

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Magnet Poles

Every magnet has two poles. The lines of

force flow from one (called the north

pole) to the other (the south pole).

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Magnet Poles

Alike poles repel each other.

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Magnet Poles

Opposite poles attract each other.

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OurMagnetic Earth

The Earthhas a magnetic field.

This can be visualized by imagining a bar

magnet inside the Earth.

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OurMagnetic Earth

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Magnetic Poles

The North & South magnetic poles are not

in the same locations as the geographic

poles.

Magnetic North 78.9N 103.8W

Magnetic South 64.5S 139.5E

The location of the poles are constantly

changing.

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Dip

The magnetic lines of force have vertical

and horizontal components.

Dip the verticalangle of lines of force.

0 at the magneticequator

90 at the poles

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Dip

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A Little Review

True North (0 T) the direction from you

to the geographic north pole; it is measuredalong your meridian.

Meridian the great circle passing through

the geographic poles and a position on the

Earth.

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Magnetic Directions

Magnetic North (0 M) the direction of themagnetic meridian that passes through your

position.

Magnetic Meridian a line of force running

between the magnetic poles, passing

through a position on the Earth.

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Magnetic Directions

Why Not?

Magnetic Directions True Directions

Geographic NorthPole is in a different

place than the Magnetic NorthPole.

Magnetic meridians arent great circles.

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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.

- Duttons

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Variation

Local variation is

shown on the

compass rose

nearest your

location.

It changes overtime.

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Variation

Labeled West (W)

if 0M is west (or

to the left) of 0T.

Labeled East (E) if

0M is east (or tothe right) of 0T.

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Introduction to Navigation

Questions?