Compass

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Imagine that you are in the middle of the ocean, and you are looking all around you in every direction and all you can see is water.  It is overcast so you cannot see the sun.  How would you know which way to go unless you had a compass to tell you which way is North?

In order to find direction, early map makers drew a small 16-pointed circle on a map.  Next, they would place an "N" to point North. These points became known as the 16 Cardinal Points from which the winds of the Earth were thought to blow. The drawing became known as a Wind Rose.  Once the magnetic compass was developed, the magnetic compass was placed on top of the Wind Rose pattern.  This helped insure that the nautical chart would face in the proper direction. In time, the Wind Rose became known as a Compass Rose.

For centuries, this was the best in accuracy for traveling the seven seas.  Obviously, it is not very accurate by today's standards.  With more information, and the improvement of spherical mathematics, it eventually became customary to give bearings in units of degrees rather than the use of Cardinal Points.  During the early part of the 20th Century, it became accepted practice to indicate direction (also called "heading" or "bearing") in terms of degrees representing the degrees of a circle as measured clockwise from True North.

No matter where you stand on Earth, you can hold a compass in your hand and it will point toward the North Pole.  Long before GPS satellites and other high-tech navigational aids, the compass gave humans an easy and inexpensive way to orient themselves.  While you might think the compass is no longer useful thanks to our GPS satellite system and handheld GPS indicators, you would be wrong.  People use the compass, along with a map, in the sport called Orienteering.  Besides, it is important to learn how to use a compass in the event some alien civilization knocks out our GPS satellites!

The compass is an extremely simple device.  A magnetic compass consists of a small, lightweight magnet balanced on a nearly frictionless pivot point.  The magnet is generally called a needle and one end of the needle is colored in some way to indicate that it points toward north.

The reason why a compass works is this: think of the Earth as having a gigantic bar magnet buried inside. In order for the north end of the compass to point toward the North Pole, you have to assume that the buried bar magnet has its south end at the North Pole.  Hence, the "opposites attract" rule of magnets causes the north end of the compass needle to point toward the south end of the buried bar magnet, and therefore the compass points toward the North Pole.

The bar magnet does not run exactly along the Earth's rotational axis. It is skewed slightly off-center.  This skew is called declination, and well-made maps indicate what the declination is in different areas because it changes depending on where you are on the planet.  The magnetic field of the Earth is relatively weak on the surface because with Earth being almost 8,000 miles in diameter, the magnetic field has to travel a long way to affect your compass.  That is why a compass needs to have a lightweight magnet and a frictionless bearing; otherwise, there wouldn't be enough strength in the Earth's magnetic field to turn the needle.

Obviously, there is no big bar magnet that runs through the middle of the Earth.  However, what really is happening is still a theory.  It is believed the Earth's core consists largely of molten iron.  At the very core, the pressure is so great that this super-hot iron crystallizes into a solid.  Convection caused by heat radiating from the core, along with the rotation of the Earth, causes the liquid iron to move in a rotational pattern.  It is believed that these rotational forces in the liquid iron layer lead to weak magnetic forces around the axis of spin.

A magnetic compass has several problems when used on moving platforms like ships and airplanes.  It must be level, and it tends to correct itself rather slowly when the platform turns.  As a result, ships and airplanes use gyroscopic compasses.  A spinning gyroscope maintains the direction it is pointing.  In a gyrocompass, this tendency is used to emulate a magnetic compass.  At the start of the trip, the axis of the gyrocompass is pointed toward north using a magnetic compass as a reference.  A motor inside the gyrocompass keeps the gyroscope spinning, so the gyrocompass will continue pointing toward north and will adjust itself swiftly and accurately even if the boat is in rough seas or the plane hits turbulence.  Periodically, the gyrocompass is checked against the magnetic compass to correct any error it might pick up.

Obviously, this is old technology.  With our Global Positioning System (GPS) satellites in orbit around the Earth, it has become much easier to pin-point location.  The GPS consists of 24 Earth-orbiting satellites.  These satellites allow any person who owns a GPS receiver to determine his or her precise longitude, latitude and altitude anywhere on the planet.

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