The needle never lies. It just answers a different question.

Quick answer: A compass needle is a small magnet that lines up with Earth's magnetic field. That field points toward a spot in the Arctic that isn't the North Pole, so the needle is off from true north by a few degrees or a few dozen, depending on where you stand. Metal and magnets nearby can throw it off further.

What's pushing the needle

About 1,800 miles under your boots, Earth's outer core is a layer of liquid iron and nickel, hot enough to glow and big enough to swirl. As it churns, and as the planet's spin twists the flow, it drives electrical currents. Electrical currents make magnetic fields. The result is a field that reaches far out into space and passes through your backyard.

Scientists call this the geodynamo. Nobody has drilled down to watch it run, so what we know comes from physics, seismic waves and decades of field measurements. It's a good theory. It's also a working one, because the field it describes is exactly what your compass reads.

Now the needle. It's a sliver of magnetized steel balanced on a nearly frictionless pivot, often a tiny jewel. Earth's field gives it a gentle twist until it lines up with the field. That's all. The needle isn't seeking anything. It's a lever being turned by an invisible hand.

One oddity is worth knowing. Opposite magnetic poles attract, and the end of your needle marked north is the end that swings toward the Arctic. So the magnetic pole up there is, by the rules of magnets, a south pole. Every compass in the world is quietly built on that backwards-sounding fact.

Lie number one: magnetic north isn't true north

True north is the point your planet spins around. Magnetic north is where the field lines plunge into the ground, and it sits somewhere else. The angle between the two, measured from where you stand, is called declination.

It's not a small error in most places. In coastal Maine the needle points roughly 14 degrees west of true north. In Washington State it points roughly 15 degrees east. Somewhere in between, it lines up with true north and the error disappears. Walk a straight line for a mile with 15 degrees of error, and you'll finish a quarter mile or so off your target.

And it keeps moving. The magnetic pole has been drifting toward Siberia, at its fastest more than 30 miles a year. That's why the government's World Magnetic Model gets revised every five years, and why the declination printed on an old map can be wrong. Check the date in the margin.

Pilots deal with the same thing. Runways are numbered by magnetic heading, so when the drift adds up, airports renumber them. If you've read how airplanes actually stay in the air, this is the quieter half of the story: the aircraft isn't just held up, it's pointed by a field nobody can see.

The fix is simple. Look up the declination for your area from a current source, then set it on your compass if it has an adjustment screw. Or just write it on a piece of tape on the baseplate and do the arithmetic. Pick one and stick with it.

Lie number two: the metal around you

Your compass doesn't know the difference between Earth's field and a field from a nearby chunk of steel. Earth's field is weak. A fridge magnet beats it by a wide margin. So anything with iron in it, or anything magnetic, can win the tug of war against the planet.

Here's the common scene. You set the compass on your truck's hood to check a bearing. The needle settles and shows you something wrong, and you don't know it's wrong. The engine block and the steel body are doing the talking.

Keep it clear of:

• Vehicles, including the tailgate, the hood and a gun safe in the back seat
• Your belt buckle, knife, rifle and trekking poles
• Your phone, especially a case with a magnetic clasp
• Power lines, fences and buried pipe
• Rocks rich in iron ore, which in some regions can pull a needle several degrees by themselves

Stand a few paces from your truck, hold the compass level, and read it away from your gear. If the reading changes when you shift your feet, something nearby is interfering.

Phones are worth a note. Your phone has a tiny electronic magnetometer that does the same job as the needle, and it has the same weakness. That's part of why the little cone on a map app can swing around when you walk next to a steel building. It's a cousin of the problem behind why your blue dot jumps downtown, though that one is mostly about GPS signals bouncing off buildings.

Lie number three: the tilt

Earth's field doesn't run flat along the ground. It comes in at a slant, and across the lower 48 that slant is steep, pointing well down into the ground. Only the horizontal part of it turns your needle. The downward part just tries to drag one end of the needle toward the dirt.

Compass makers deal with this by balancing the needle for a particular region of the world. A compass built for North America will work fine from Alaska to Florida. Carry it to Australia and the needle will drag against the housing and stick, because the field there tilts the other way. If you travel far, buy a compass built for global use or one made for the region you're going to.

The tilt also explains why a compass gets sluggish near the magnetic poles. Up there the field points almost straight down, so the horizontal push on the needle fades to almost nothing. At that point the needle has nothing to say.

Lie number four: the needle itself

A compass can be wrong because its own magnetism got flipped. Store it next to a speaker, a magnetic knife holder or a strong magnet and the needle can reverse, so the end labeled north now points south. It's easy to miss because the needle still settles and looks calm.

Check it before every trip. The red end should point toward the north side of the sky, and if you aren't sure which way that is, use the sun or Polaris to confirm. Polaris sits within about a degree of true north, which makes it the best free check you'll get. Remember to correct for declination if you're comparing it with the needle.

A two-minute test you can do anywhere

You don't need a clear night. On a sunny day, push a stick straight into level ground and mark the tip of its shadow with a stone. Wait 15 minutes. Mark the tip again. The line from the first stone to the second runs roughly west to east. Stand with the first stone on your left and the second on your right, and you're facing roughly north.

Now lay the compass beside that line, away from your truck, your phone and your belt. If the needle agrees after you correct for declination, you can trust it. If it doesn't, find out why before you head into the woods.

What the needle is telling you

The needle is honest. It points exactly where the field points, every time, with no battery, no signal and no subscription. A man who knows how it works can find his way home with a piece of steel and a little arithmetic.

The trouble is that "where the field points" and "north" are two different questions. Learn the difference and the compass stops lying to you.