A magnetic compass doesn’t usually point to true north because Earth’s magnetic field and its rotation axis are not aligned. The north-seeking tip of the needle settles along the horizontal direction of the local magnetic field. The angle between that direction and geographic north is called magnetic declination. Its size and direction vary from place to place and change over time.
What is the difference between true north and magnetic north?
True north is the direction along Earth’s surface toward the geographic North Pole, where Earth’s axis of rotation meets the surface. Magnetic north is the direction a magnetic compass points when it is functioning normally and away from interference. The compass isn’t directly “looking for” the moving north magnetic pole on a map; it is responding to the field where you stand. That’s an important distinction in regions where the field curves or inclines sharply.
How does the needle actually line up?
The compass needle is a small magnet balanced so it can rotate. Earth’s magnetic field exerts a torque on it, bringing it into alignment with the field’s horizontal component. In an ordinary handheld compass, one end is marked to help you identify the north-seeking direction. If you turn the housing, the magnetic needle can still indicate the magnetic direction as long as it has room to move.
How do you correct for magnetic declination?
First check the declination for your location and date, not a generic value taken from a distant city. NOAA’s magnetic declination information and calculators explain the convention: east declination is positive, west declination is negative. In that convention, the true bearing equals the magnetic bearing plus the signed declination. For example, if your compass indicates 40° and the local declination is 8° east, the true bearing is about 48°; with 8° west it would be about 32°. For actual field navigation, follow your particular compass’s adjustment instructions so you don’t correct the same angle twice.
Can a compass point the wrong way indoors?
Yes. Nearby magnets, large steel objects, electric currents, vehicle interiors and some electronic equipment can disturb a magnetic needle. Try moving away from those items, holding the compass level and waiting for the needle to settle. A phone’s digital compass also needs calibration and can be affected by magnetic accessories. For safety-critical hiking, use an appropriate map, verify bearings with landmarks and avoid relying on a single device.
Does magnetic declination stay the same forever?
No. Earth’s magnetic field changes gradually as flows of electrically conducting liquid iron in the outer core evolve. Local declination changes accordingly. That’s why older maps may show a declination value and year that are no longer current. The U.S. Geological Survey explains why declination varies across locations and time.
Frequently asked questions
Does a compass always point north?
A properly functioning magnetic compass generally lines up with the local horizontal magnetic field. It may not align with geographic north, and interference can make its reading unreliable.
Is declination the same as magnetic inclination?
No. Declination describes the horizontal angle between magnetic and true north. Inclination, or magnetic dip, is the angle the field makes above or below the horizontal plane.
Can I use a compass without cell service?
Yes. A traditional magnetic compass needs no battery or network. You still need navigational skills and an appropriate map to use it effectively.
For a practical next step, read Wondelique’s beginner orienteering guide or learn how to read latitude and longitude.
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