Three quick flashes
Cardinal mark light at night. Developing level, about 2 min. IALA cardinal mark lights.
The situation
Night. Ahead, a white light is flashing near a charted danger. You cannot make out the buoy. Press play, watch the light for a full cycle and count the flashes.
What kind of mark is this, and where is the safe water?
- East cardinal: pass east of it: Quick (3) every 10 seconds.
- South cardinal: pass south of it: Quick (6) plus a long flash every 15 seconds.
- Isolated danger: give it a wide berth: Group flashing (2).
Lookout: Cardinal lights follow a clock face: east is three o’clock, three flashes; south six, west nine. North flashes continuously.
The sound answer
East cardinal: pass east of it
Three quick flashes followed by darkness, repeated every 10 seconds, is the light of an east cardinal mark (or three very quick flashes every 5 seconds). The safe water lies to its east.
Why the other options fall short
South cardinal: pass south of it
A south cardinal shows six quick flashes followed by one long flash. You saw three flashes and no long flash.
Isolated danger: give it a wide berth
An isolated danger mark shows two flashes in a group, and slower. Three quick flashes are the east cardinal character.
Cardinal mark lights
Cardinal lights are white, quick or very quick. North: continuous. East: three flashes in a group. South: six flashes followed by a long flash. West: nine flashes. Quick is 50 to 79 flashes a minute, usually 60.