Fathom isn't a bearing: understanding true, magnetic, and relative bearings in seamanship

Navigating by bearings: the quiet trio every mariner should know

Let’s imagine you’re on watch, the horizon ribbing into a quiet blue day, and a lighthouse tick-tocks in the distance. You don’t just point and go. You measure. You compare. You keep your wits about you with a simple trio of ideas called bearings. In the world of seamanship, bearings are the compass of direction. They tell you where a thing is, relative to something else. And yes, they’re essential no matter how fancy your chart plotter or radar gets. Here’s the thing: among the three types of bearings that sailors actually use—true bearing, magnetic bearing, and relative bearing—only one of the options in a common quiz is not a bearing. The odd one out is fathom. Fathom is about depth, not direction. Easy to miss, but it matters.

What a bearing really is, and why three kinds matter

Think of a bearing as a directional line from you to an object, but with different reference points to measure from. If you snap a mental line to the object and then read the angle from a reference line, you’ve got a bearing. It sounds technical, but it’s something you use in the real world all the time. The three bearings you’ll encounter in most seamanship contexts are:

• True bearing: The angle in degrees from true north to the object. This one is absolutist — it doesn’t care about magnetic fields or the ship’s own heading. It’s like aiming at a star and trusting that line up there in the sky stays constant.

• Magnetic bearing: This is the angle from magnetic north to the object. Magnetic north isn’t the same as true north because the Earth’s magnetic field isn’t a perfect, fixed arrow. The difference between them is called magnetic variation (and ships sometimes have to account for compass deviation inside their own hull). So magnetic bearings shift depending on where you are on the planet.

• Relative bearing: The angle from your ship’s current heading to the object. If your bow (the front of the ship) is pointing north and a buoy sits on your port bow, the relative bearing is the angle you’d measure from the bow to that buoy. Relative bearing is what you actively act on when you’re steering to keep something in sight or avoid a collision.

In these three, you’re always talking about direction on the surface of the Earth. A distance measurement might come from depth soundings or range, but the bearings themselves are about where you’re aiming or looking.

Why fathom doesn’t fit in

Now the quiz question you’ll see in many nautical glossaries asks which of the following is not a bearing: True Bearing, Magnetic Bearing, Relative Bearing, or Fathom. The correct answer is Fathom. A fathom is a unit of depth—six feet, to be precise. It’s the way sailors describe how deep the water is, not which way you’re pointing. Depth and direction live in the same nautical world, but they occupy different rooms in the ship’s mental toolbox. When you’re plotting a course or navigating around shoals, you care about the bearing to a landmark or another vessel. When you’re avoiding a snag or figuring how much rope to pull in on an anchor, depth is the currency. It’s a subtle distinction, but crucial for safe navigation.

True bearing: reading the sky and the chart alike

Let me explain true bearing in practical terms. Suppose you sight the lighthouse on the headland at your six o’clock position. If you could draw a line from your ship to the lighthouse and measure the angle from true north to that line, you’ve got a true bearing. This is an absolute direction—unless you drift to another latitude or the Earth wobbles a bit (which it does, but not in the few minutes you’re plotting). The advantage of true bearing is its independence from local magnetic quirks. It’s your north-star-like direction: stable, consistent, and easy to compare across charts and national specifications.

Of course, detecting true north in the real world isn’t always straightforward. You don’t carry a giant compass marked “True North” on your deck. Instead, you rely on celestial sights in the old-school sense, or you use a gyrocompass, which provides a true-north reference by sensing the Earth’s rotation rather than magnetic fields. In practice, most sailors translate true bearings into a course line on the chart, then use that line to plot a course or to determine a fix. It’s the backbone of accurate navigation, even if modern gadgets do most of the heavy lifting behind the scenes.

Magnetic bearing: when the Earth’s magnet plays tricks

Magnetic bearing brings in the magnetic north you learned about in clocks-and-compasses class. When you measure from magnetic north to an object, you’re dealing with a moving target—because magnetic north isn’t fixed. It shifts with location and even with time. That’s why a navigator carries a chart showing magnetic variation for the port and adjusts accordingly.

In the real world, you’ll often see magnetic bearings used with a magnetic compass on deck. If your compass points 5 degrees to the east of true north at your location, and the object is due east on the chart, the magnetic bearing would differ from the true bearing by that few degrees. Add ship’s own magnetic deviation — some ferrous equipment inside the hull pulls the compass slightly off its true reading — and you can see why seasoned mariners cross-check with multiple sources. The more you work with magnetic bearings, the more you begin to appreciate the art of correction: variation plus deviation, filtered through a careful eye on the horizon.

Relative bearing: steering by what’s in view, not what’s on the map

Relative bearing is perhaps the most immediately useful on watch. It’s the angle from your ship’s heading to the target. If you’re steaming southeast at 120 degrees and a vessel to your stern-away point sits at a 50-degree angle from your bow, the relative bearing is 50 degrees. If you turn to a new heading, that relative bearing shifts, and your action might be to steer so that the other vessel remains at a safe angle, or to keep a buoy in sight as you pass it on your starboard side.

Relative bearing is the practical partner to the ship’s radar and the bridge watch. It’s the number you’ll translate into a course change, a target bearing in a collision-avoidance scenario, or a line of sight for a visual fix. It’s also how you coordinate with the crew: “Maintain 45 degrees relative to the bow” is a way to keep your plan clear and everyone aligned. The key is to recognize that relative bearing is not a fixed property of the world; it changes as your ship’s heading changes.

A quick sailor’s primer: how to think about bearings in the field

• Visualize the reference: True north is the fixed reference line in the sky. Magnetic north is a magnetic ray that shifts. Relative bearing is a moving line tied to your ship’s current heading.

• Cross-check often: If you’re ever unsure about which bearing you’re using, ask yourself, “What reference am I reading from?” Then double-check with the chart, the compass, and the radar.

• Practice with simple examples: If a buoy lies on your bow, your relative bearing is 0 degrees. If the buoy is dead ahead on your true-north line, the true bearing is whatever the lighthouse angle is from true north. If the same buoy shows up as 10 degrees to your starboard on the compass, that’s a magnetic bearing with deviation factored in.

• Don’t confuse depth with direction: A fathom is a depth unit. It’s six feet of water. It keeps you safe, helps you anchor, and informs your passage near shoals. It sits in your depth-measurement toolkit, not in your heading toolkit.

How bearings fit into the bigger picture of seamanship

Bearings aren’t just abstract numbers. They’re the language sailors use to describe where things are in relation to the ship, and they tie directly into course plotting, position fixes, and safe navigation. Here’s how they weave into the daily flow on deck:

• Position fixes: When you take bearings to known landmarks from at least two positions, you can triangulate your exact location on the chart. It’s the classic drift-and-use-your-eyes method that still has plenty of life in it, even with GPS around.

• Collision avoidance: A constant awareness of relative bearing to other ships lets you estimate closing speed and decide on evasive action or a change of course early.

• Anchoring and harbor work: Knowing the depth (fathoms) helps you pick the right anchor and ensure you’re in water deep enough for the chain to hold. Depth stories often share the deck with directional ones, especially near channels or shoals.

• Weather and currents: Bearings help you plan against currents and wind drift. A good navigator always asks, “Where is this ship pointing, and how will that affect my course in a changing sea?”

A couple of practical tips you can try on a calm day

• Match the bearings to the hardware. Use the gyro compass for true bearings and your magnetic compass for magnetic bearings, but always be mindful of deviation and variation. If your ship’s systems disagree, note the discrepancy and investigate with the charts.

• Keep a simple notebook: write down the relative bearing to a fixed landmark at regular intervals. See how the numbers shift as you steer. It’s a tiny habit, but it builds intuition.

• Use a mental checklist: heading, target bearing (true or magnetic, whichever your plot requires), and a quick glance at the radar to confirm the object’s position. It’s not fancy, but it works.

A little something to remember next time the sea says hello

Three bearings, one calm approach: true bearing anchors you in a stable reference, magnetic bearing morphs with the Earth’s magnetism, and relative bearing keeps you in tune with what your ship is doing. Fathom, the depth unit, reminds you that a voyage isn’t only about where you’re going, but how deep the water sits beneath your keel. Both sets of numbers—directional and depth—shape safe, steady sailing.

If you’re ever tempted to view navigation as a high-tech sport, remember the roots. Early sailors relied on sight lines, simple compasses, and the patience to read a chart. That same discipline—knowing which bearing you’re using, and why it matters—still serves today, even with all the gadgets on deck. Bearings are the language you use to talk with the sea, and the sea always answers back.

A quick recap with a friendly glance backward

• True bearing: angle from true north to an object; absolute and consistent.

• Magnetic bearing: angle from magnetic north; varies with location and ship’s own deviations.

• Relative bearing: angle from the ship’s heading to the object; used for steering and collision avoidance.

• Fathom: a depth unit, not a bearing. Six feet of water, the measure of how deep the sea is beneath you.

• In real-world sailing, you switch between these references as you plot, sail, and watch. The trick is to stay aware of which reference you’re using at any moment.

So next time you’re on deck and a buoy catches your eye, or a lighthouse points its steady glow across the waves, you’ll know exactly which bearing you’re reading. You’ll also know that depth matters—because a safe voyage isn’t just about pointing in the right direction, it’s also about knowing when not to push too far into shallow waters. Bearings, after all, are about staying on course while keeping your feet (and hull) firmly on the right path.