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The circle is one of the three basic shapes in geometry. A compass is a circular device used for many purposes, but we know it primarily for its use in navigation. Ships at sea and airplanes in the air need a way of determining direction. A magnetic compass is designed so that the needle points North. The other directions are known in relation to North.


For a compass to be a useful instrument, it must measure any direction. Ancient Babylonians would measure the movement of the earth in relation to certain stars. It was thought that it took 360 days for the earth to make one complete revolution around the sun until the star was visible in the same position again. Therefore the Babylonian calendar had 360 days.
The circle was divided into 360 equal parts, each part called a degree. Consequently, the modern compass has 360 degrees.

Airplanes and ships have relied on the compass for directional information (called a "heading") for many decades.

However, today most airplanes use special ground based radio transmitters which are designed to aid air navigation. These transmitters are called VOR stations. On an aerial navigation map, the VOR station is indicated by a small hexagon with a compass around it.. (VOR stands for Very High Frequency Omni Range which is a technical, electronic description of the transmitter.)

The aircraft pilot may choose the magnetic heading desired to fly to or from a VOR station. This heading is now called a course because it will be along a specific path over the ground. The exact magnetic direction which is flown away from a VOR station is called a radial. This term (radial) is used because the navigational radio signal is being "radiated" out from the station in a specific magnetic direction.

In the example above, the airplane's course (or heading) is on the 300 degree radial away from the VOR. If the airplane were headed the opposite direction, toward the VOR, it would still be on the 300 degree radial from the VOR, but the plane's course would be 120 degrees.

Knowledge Review - Basic Compass Knowledge
Knowledge Review - Compass Knowledge of the United States
Knowledge Review - Compass Knowledge of Europe and Africa
Knowledge Review - Compass Knowledge of North and South America


Clip1.gif (22753 bytes)    The magnetic compass used in airplanes is a simple self contained instrument.  A "compass card" is mounted on a floating ring which has two magnetized needles, which always point  to magnetic North.  The compass card has letters for cardinal headings, that is N (north), S (south), E (east) and W (west).  Each 30 (30 degrees) interval of direction is represented by a number from which the last zero is omitted.  Between the numbers, the card is graduated for each 5 (5 degrees). 
For  example,on the compass 240  would look like 24.  On the left, the compass indicates 96 (E is the same as 90, the line to the left of E is a 5 line and the vertical line is a little to the left of that line).

Knowledge Review - Magnetic Compass and Compass Reading


Construct one of the simple compasses as described in the two following experiments:

Experiment 7 - Dry Compass

Experiment 8 - Wet Compass


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Updated: March 12, 2004