Have you ever gotten lost in the outdoors, or had trouble finding your way with a map? Do you and your children know how to use a compass to find your way? In many situations, compasses are essential for finding direction.
Compasses measure direction in degrees, which on a compass increase clockwise; East is at 90 degrees, South at 180, West at 270, and North at 360 or 0 degrees. If you plan to use a compass frequently, we suggest you have an orienteering compass. These are accurate and easy to use, without being too expensive. Use your compass to find the bearing, or location of a direction in degrees, of a landmark (such as a large tree, a building, or mountain). If you have an orienteering compass, point the arrow on the base plate toward the landmark. Then turn the dial of the compass until North is aligned with the red tip of the compass needle. The end of the arrow will be pointing to a number in degrees on the dial; this is the bearing for the landmark.
Compass needles are magnetized, so that one end will always point to magnetic north. There's more to 'north' than you might know about, though! Besides magnetic north (the magnetic north pole) there is 'true' or geographical north. Maps are drawn so that any bearing taken from them is based on true north and is therefore a true bearing. But here's where the tricky part comes in: compass needles point to magnetic north, which does not match up with true north. Because of this, if you use a map to find your bearing you will need to calculate declination.
Declination is the angle between true north and magnetic north. Whatever map you're using should tell the declination for that area in degrees. There is an imaginary line running through true north and magnetic north, so that the declination along this line is at zero; it is called the agonic line. If you are to the west of this line, in the western United States, then the compass needle leans slightly too far east, to the magnetic north pole. The declination should be subtracted from the true bearing of the map, in order to find the bearing you should use with your compass. Thus, if you calculate a bearing of 290 degrees from a map and the declination is 12 degrees, you will need to subtract the declination to get the magnetic bearing (278 degrees) to use with your compass. If you are on the other side of the agonic line, in the eastern United States, so that your needle points too far to the west, you will need to add the declination to get your compass bearing. Washington and Maine have the highest declination in the U.S., both at 20 degrees. The agonic line runs approximately from Wisconsin through the tip of Florida.
For more fun with compasses, try out the ideas in our Compass Activities Science Project.