Navigation NAU 102 Lesson 14

Magnetism & Compasses A basic function of navigation is finding direction. We must determine: Courses Headings Bearings

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?