Third Mate · ONC02 · Deck · Navigation Lights · CURRENT-Q100-Q0033
Sternlight Arc: 67.5° Each Side Astern
Read the navigation-light sectors and distinguish the total sternlight arc from the angle on each side of dead astern.
BOTH INTERNATIONAL & INLAND The stern light shall be positioned such that it will show from dead astern to how many degrees on each side of the stern of the vessel?
- A. 22.5°
- B. 67.5°
- C. 112.5°
- D. 135.0°
Correct answer: B. 67.5°
Why this answer is correct
A sternlight shows 67.5 degrees on each side of dead astern.
Governing principle
Rule 21(c) defines a 135-degree total arc centered on dead astern.
Reasoning
- The total sternlight arc is 135 degrees.
- It is centered on dead astern.
- Half of 135 degrees is 67.5 degrees.
Why the other choices do not fit
A. 22.5 degrees is not half of the sternlight’s 135-degree arc.
C. 112.5 degrees is the arc of one sidelight, not one side of a sternlight.
D. 135 degrees is the full sternlight arc, not the angle on each side.
Related lesson
Navigation Lights, Shapes & Identity Signals — Arcs, Vessel Status, Tows, Public Safety and Small Craft
Flashcard preview
What are the basic arcs of masthead, sidelight and sternlight?
Masthead light 225°, each sidelight 112.5°, and sternlight 135° total—67.5° on each side of dead astern.
Sources
- USCG Navigation Center · Amalgamated Navigation Rules · Rule 21(c), Definition of Sternlight
- USCG National Maritime Center · Q100 Rules of the Road: International and Inland · Question 33 and official answer key
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