How far can a stern light be seen? A clear look at the 135-degree visibility under COLREGs

Outline

• Set the scene and pose a practical question about stern lights.

• Explain the stern light arc: 67.5 degrees to either side, total 135 degrees.

• Connect the angle to safety on the water and how other vessels interpret what they see.

• Briefly mention how stern lights fit with other vessel lights.

• Add a real-world feel with a light digression and relatable analogy.

• Close with a concise recap and a practical takeaway.

A clear, night-time signal: why the stern light angle matters

Picture this: you’re on a calm midnight glide, a faint spray of stars above, and the water breathing back a quiet hiss. Off the stern, a vessel’s white light glows. You don’t need a telescope to notice it, but you do want to know what its reach is. How wide is that stern light visible, really? If you’ve ever wondered exactly how far behind a boat that light can be seen, you’re in good company. The answer, straight from the COLREGs, is 135 degrees.

Here’s the thing about the stern light. It should be visible from behind the vessel and out toward its rear corners—specifically, it covers 67.5 degrees to the left of the centerline and 67.5 degrees to the right. Do a quick sum in your head and you get 135 degrees of visibility. It’s a neat, precise slice of the night, designed on purpose so nearby boats can read a vessel’s length and direction even when they’re looking from behind.

Why that angle matters in real life

Navigation at sea isn’t a one-spot game. It’s a continual, rotating puzzle where every glint of a light is a clue about who’s where and where they’re heading. The stern light is a beacon that helps other mariners recognize a vessel’s rear and its movement, especially when visibility is limited. If you’re following a ship or if you’re the one behind it, that arc tells you, “This is a vessel moving forward, with its stern roughly in your line of sight.” When you’re skirting along a fog bank or night breeze, every degree counts—135 of them in this case.

Think about it this way: if the stern light were visible a full 180 degrees behind you, it would muddy the picture. Or if it were visible only straight back (a tiny slice), other boats wouldn’t realize how long your vessel is or how you’re aligned with their path. The 135-degree range keeps a reliable, predictable signal in night conditions, which is exactly what you want when a quick, correct interpretation can prevent a collision.

How stern lights fit into the whole light show on the water

The stern light is one piece of a larger, coordinated lighting system. You’ve got the white stern light, the red port sidelight, and the green starboard sidelight, plus a masthead light on many vessels. Each light has its own arc and purpose, contributing to a two-key message: what you are (a vessel), and how you’re moving (direction, speed, and heading for the next few moments).

• The stern light’s job is to signal the rear aspect of the vessel’s presence and its orientation to anything approaching from behind.

• The sidelights outline the vessel’s sides and help others gauge its course relative to forward movement.

• The masthead light, when required, gives a sense of a vessel’s size and continuing trajectory on the open water, especially at greater distances.

Put simply: the stern light communicates “I’m moving forward, and I’m here,” while the other lights add the shape and distance of the whole vessel to the scene. That combination keeps navigation readable, even when visibility is less than ideal.

A small detour you might enjoy

If you’ve ever watched a harbor from shore at night, you might notice how the lights on different boats form a kind of celestial traffic jam. The arcs aren’t random; they’re engineered so that as boats cross and pass, the other mariners can piece together who’s where and what they’re likely to do next. It’s a little like reading a crowd at a festival—everyone’s signals are compact and well-defined, so nobody trips over one another. The stern light arc is one of those quiet, dependable signals that keeps the waterway orderly.

Putting the 135-degree rule into practical view

Let’s ground this in a quick, practical way you can apply on the water. When you scan for traffic at night, you’re looking for patterns in lights:

• If you see a white light at the stern with a broad arc toward the sides (roughly 135 degrees), you’re looking at a vessel’s rear. You can infer that vessel is oriented to move forward, with the stern in your line of sight.

• If you see the red and green sidelight combination, you’re seeing the vessel’s sides coming toward you or away from you, which helps deduce its relative position and path.

• A masthead light adds distance context, indicating a larger vessel or a vessel with higher clearance.

Meld all that together and you start getting a mental map that helps you decide who should alter course, who might be the stand-on vessel, and who’s the give-way candidate. The 135-degree stern-light rule is the hinge that keeps that mental map accurate when the night is thick with mist or the sea carries a little extra glare.

Common misunderstandings—and the clear answer

You might have heard different numbers tossed around, or wondered if the stern light somehow reaches all the way to 180 degrees behind. Here’s the crisp takeaway you can rely on:

• The correct visibility arc for the stern light is 135 degrees. That’s 67.5 degrees to the left of the vessel’s centerline and 67.5 degrees to the right.

• The arc is designed to be seen from behind, not forward, which is why you don’t see it head-on.

• This 135-degree specification is part of the COLREGs’ broader system of lights that communicate a vessel’s identity, size, and direction at night.

If you ever feel unsure, remember the mental picture: a white light behind the boat, sweeping a 135-degree swath across the rear half of the horizon. It’s precise enough to be practical, but simple enough to remember after a long watch.

A quick, friendly check-list for sailors and students

To keep the concept handy on deck or in study notes, here’s a compact reminder:

• Stern light: white, 135-degree arc (67.5 degrees to each side of the centerline). Visible from behind.

• Sidelights: red on the port side, green on the starboard side. They help define the vessel’s sides and forward path.

• Masthead light: visible from a distance, signaling the vessel’s forward orientation and size.

• Use the arcs together to gauge a vessel’s position, direction, and potential conflict path. If you can’t see the stern light from behind, you might be too close to the other vessel’s beam or there could be an obstruction.

A touch of realism: why this matters for you on the water

If you’ve ever run a boat at night, you know how quickly conditions can change. A bank of fog can roll in, the harbor lights bounce off a calm sea, and suddenly every light looks a little different. In those moments, it isn’t about memorizing numbers. It’s about recognizing predictable signals you can rely on. The stern light’s 135-degree arc is precisely that: a dependable cue that helps you interpret a nearby boat’s path when you’re sharing the same stretch of water.

Emotional note and practical takeaway

There’s a certain quiet confidence that comes from understanding what you’re looking at in the dark. It’s not flashy or dramatic, but it matters. When a vessel’s stern light reaches you at 135 degrees, you’re reading a story about its momentum and intent. You’re not guessing; you’re using a signal with a long track record of working in real conditions, night after night, coastlines after coastlines. That clarity is what keeps people and boats safe—day or night.

A final recap you can tuck into memory

• The stern light’s visible arc is 135 degrees total: 67.5 degrees to each side of the centerline, behind the vessel.

• This design makes it possible to gauge a vessel’s rear orientation and speed from a fair distance, even in imperfect conditions.

• In practice, read the entire light configuration—the stern light, sidelight colors, and any masthead signals—to form a complete sense of where other boats are and where they’re headed.

If you’re curious, you can always relate this back to a simple, everyday analogy: think of the stern light as the rear window of a car—only here it’s a carefully shaped, sea-tested arc that helps prevent collisions by giving other mariners a clear hint about where you’ve come from and where you’re going next. On the water, as on land, clear signals beat guesswork every time.

So next time you’re scanning the horizon after the sun has dipped, you’ll know exactly what that white light behind you is telling you—and you’ll have a better sense of how to stay safely out of each other’s way. The 135-degree stern-light rule isn’t just a number; it’s a practical tool that helps keep the night sea a little less chaotic and a lot safer for everyone aboard.