Riddle #223

A warship's captain radios a light dead ahead on a collision course and orders it to alter course 15 degrees north. Back comes: 'Alter YOUR course 15 degrees south.' Furious, the captain radios that he commands the largest ship in the fleet and demands they give way. The reply: 'I am a lighthouse.' Who moves?

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Open-ended riddle: think it through, then reveal the answer.