Reading a Vessel's Aspect at Night From Its Navigation Lights

There is a particular unease that settles on a new night watch-keeper when a cluster of lights appears on the horizon and refuses to resolve into anything obvious. Is that vessel crossing ahead, heading straight for you, or already past? The answer is coded into the colours and arcs ahead of you, and knowing the grammar makes all the difference.

The International Regulations for Preventing Collisions at Sea assign specific arcs of visibility to each light type, and those arcs are not arbitrary. They are engineered so that, from any direction, an observer can deduce aspect and therefore stand-on or give-way obligation. Getting that read right is not an exam nicety; it is the foundation of every collision-avoidance decision you make after dark.

This article walks through the arc geometry, the combinations you will actually see, and the misreads that trip up even experienced mariners. It is educational support intended to sit alongside proper training and the official publications, not to replace them.

The Building Blocks: Arcs of Visibility

Three arc sizes govern almost everything. Rule 21 defines the lights and their sectors:

• Sidelights each cover 112.5 degrees - the green on the starboard side, the red on the port side. Each arc runs from dead ahead to 22.5 degrees abaft the beam on its respective side. Together the two sidelights cover the full forward hemisphere of 225 degrees.
• The sternlight covers 135 degrees, centred on dead astern, reaching 67.5 degrees either side of the centreline.
• The masthead light (a white steaming light) covers 225 degrees - the entire forward arc from 22.5 degrees abaft the beam on the port side, sweeping forward and all the way round to 22.5 degrees abaft the beam on starboard.

Notice how the 112.5-degree sidelight arc and the 67.5-degree half of the sternlight arc meet at exactly 22.5 degrees abaft the beam on each side. The arcs are designed to meet at that bearing with neither gap nor overlap; the join is exact by design, and the apparent continuity can be broken in practice only by imprecise installation or a vessel rolling heavily in a seaway.

Rule 23 covers power-driven vessels underway. A single forward masthead light is required for all such vessels. A second masthead light, positioned higher and further aft, is required for vessels of 50 metres or more in length and is optional for vessels under 50 metres. The vertical separation of those two whites is therefore your first clue about a vessel's size category.

What Each Combination Tells You

Once you have the arc geometry in your head, reading aspect becomes a process of elimination.

Red only: You are inside the port sidelight arc, somewhere in the 112.5-degree sector to the port side of the vessel. That means you are on her port side. Under Rule 15, when two power-driven vessels are crossing, the vessel which has the other on her own starboard side is the give-way vessel. Because you are on her port side, she has you on her port side - she is therefore the give-way vessel. You are the stand-on vessel. Maintain your course and speed.

Green only: You are inside the starboard sidelight arc. That means you are on her starboard side. Under Rule 15, she has you on her starboard side, so she is the stand-on vessel and you are the give-way vessel. Alter course and/or speed to keep clear.

White only: You are seeing her sternlight. She is ahead of you and moving away, or at least presenting her stern. This is usually the most benign situation, but do not relax entirely because the vessel could be turning.

Red and green together, no white: Classic head-on. You are looking at her bow from within the forward overlap of both sidelight arcs. Rule 14 applies: both vessels alter to starboard.

Red or green plus a white masthead light above: The vessel is angled - bow-on enough to show a sidelight but not a pure head-on. The masthead light appears above and slightly displaced from the coloured light. This is a crossing situation, and the give-way logic from the red-only and green-only cases above applies.

Two whites vertically separated plus a coloured light: A power-driven vessel of 50 metres or more. The geometry of the two masthead lights tells you about the angle of approach. When they appear in line, she is heading directly at you. As the forward (lower) white moves to the left of the after (higher) white, her bow is swinging to your left - she is crossing from your right to your left. When the lower white sits to the right of the higher white, her bow is pointing to your right.

Sailing Vessels and Additional Lights

Rule 25 deals with sailing vessels under sail, which show sidelights and a sternlight but no masthead steaming light. At distance, two coloured lights with no white above strongly suggests sail. Some sailing vessels display a combined tricolour at the masthead instead - legal only on vessels under 20 metres and only when under sail alone. The tricolour incorporates red, green, and white sectors in the same arc geometry, but the white sector functions as the sternlight, not a masthead light. Do not confuse it for a powered vessel.

Sailing vessels may also show an optional red over green at the masthead - two all-round lights to make them more conspicuous. These are purely supplementary; they do not replace the required sidelights and sternlight.

For a deeper look at how the rules tie aspect to give-way obligation, the IRPCS rule library at /#learn walks through Rules 12 through 17 with worked scenarios that complement this geometry.

Common Misreads

Conflating sternlight white with masthead light white. A white light does not automatically mean head-on. A sternlight and a masthead light are both white, but their arcs are opposite. If you see only white and no coloured lights, and the white is relatively low and dim, you may be looking at her stern rather than her bow. Range and bearing rate will help resolve it, but the safest approach when unsure is to treat it as a developing situation and plot carefully.

Sidelight flicker in a seaway. A vessel rolling 5 or 10 degrees can briefly extinguish one light or flash another. A sidelight flickering in beam seas is not necessarily a vessel turning. Confirm aspect change with a steady bearing change before taking drastic action.

Assuming one masthead light means small. Vessels under 50 metres are required to show only one masthead light, but that category includes a great many commercial vessels including tugs, fishing boats, and short-sea traders. Do not underestimate size from single-masthead evidence alone.

Misreading the two-masthead geometry. The forward masthead light is lower and further forward; the after light is higher and further aft. Because the forward light is nearer the bow, the bow points toward whichever side the lower light is displaced to. When the lower light sits to the left of the higher light, her bow is pointing to your left. A useful memory aid: the low light leads, showing you where the bow is pointing.

Practical Drill: The Three Questions

Develop a habit of asking three questions whenever you pick up a light:

1. What colours can I see? Red, green, white, or a combination.
2. What whites, and how many? One masthead or two, and is there a sternlight in the picture?
3. Is the bearing changing? A steady bearing with decreasing range is a collision course regardless of aspect.

Aspect tells you the rules situation. Bearing rate tells you whether you have a problem. Both together give you the full tactical picture.

FAQ

Why do sidelights cover 112.5 degrees rather than a round number like 90 or 120? The 112.5-degree figure is derived from the old compass point system: 112.5 degrees equals exactly ten compass points. The forward hemisphere is split symmetrically, with each sidelight extending 22.5 degrees (two points) past the beam before the arc hands off to the sternlight. The geometry was fixed in the 19th century and carried forward into COLREGS because it works reliably in practice.

Can I use navigation light colours alone to decide whether to give way? Aspect from lights is your starting point, not your only input. Rule 7 requires you to use all available means to assess risk of collision - radar, AIS, bearings, and visual observation together. Lights give you a fast initial read; they do not replace systematic collision avoidance. Test your recognition speed with the light-combination scenarios at /#quiz.

What if I see red, green, and white all at once? In strict geometric terms this should not be possible from a single conventional vessel, because no single bearing falls simultaneously within both sidelight arcs and the masthead arc. If you see all three clearly, you are most likely looking at two vessels in close proximity, or a vessel with deck flooding or work lights muddying the picture. Recheck carefully and do not assume a single-vessel scenario until you are certain.

Does the sternlight arc extend evenly either side of dead astern? Yes. Rule 21 places it symmetrically, 67.5 degrees each side of the centreline measured from dead astern. The join between each sidelight and the sternlight falls at exactly 22.5 degrees abaft the beam, the same two-point margin used by the masthead light arc. The whole system is built around that consistent two-point boundary, and the arcs meet there exactly with neither gap nor overlap by design.