I believe that authority comes firstly of course from rank, and then from who has held that rank longer. So someone who has been a captain for two days will outrank someone who has just been made captain. Thus a 30yr old captain could outrank a 40yr old captain, if they achieved the rank first.
A certain person may hold a certain position (say ships captain) on a vessel. A higher ranking officer on board would technically be a guest or advisor, unless they officially pull rank and assume command.
This explains Blairs position aboard the Midway. He is a Commodore, but has no official position on board - he is merely an observer on the Midways maiden voyage. If he wanted to, he could pull rank and assume command of the Midway, put himself back on the flight roster etc., but to do so he would have to step on a number of toes.
[Edited by AzraeL on 03-11-2001 at 20:35]