Navigating by Lighthouse: Why Flash Pattern Is the Critical Information

In the age of GPS, lighthouses remain essential backup navigation aids. And the single most critical piece of information for any mariner passing a light? Its flash pattern—the unique sequence of flashes, eclipses, and colors that identifies which lighthouse you're seeing.

How Flash Patterns Work

Each lighthouse displays a characteristic—a coded signature published on nautical charts and in Light Lists. Examples:

• Fixed (F) — steady light
• Flashing (Fl) — single flash repeated (e.g., Fl 5s = flash every 5 seconds)
• Group Flashing (Gp Fl) — groups of flashes (e.g., Gp Fl(3) 10s = three flashes every 10 seconds)
• Occulting (Oc) — steady light with regular eclipses (dark periods shorter than light)
• Isophase (Iso) — equal light and dark periods
• Morse Code (Mo) — e.g., Mo(A) = dot-dash for "A"

Color Sectors

Many lights show different colors over different arcs: white for safe water, red or green for danger sectors. A bearing change from white to red means you're drifting toward hazards.

Why It Matters

At night or in fog, coastlines vanish. Multiple lights may be visible simultaneously. Without positive identification via flash pattern, you can't fix your position, avoid shoals, or verify you're in the correct channel. The 1914 Empress of Ireland disaster (1,012 dead) involved misidentification of shore lights.

Modern Context

Electronic charts (ECDIS) and AIS (Automatic Identification System) now broadcast virtual aids to navigation. But physical lighthouses—with their unmistakable, unambiguous flash patterns—remain the ultimate fail-safe when electronics fail.