In the old days, the captain's log was an official record of the ship's position for navigation. It tracked the ship's precise position every day at noon and then its course and speed for the next 24 hours. Using dead reckoning, the navigator could find the ship's position any time of day from the log.  Today, captains keep log books for different reasons. On most recreational boats, they are kept like travel diaries of the journey. Professional mariners, however, still track their time at sea to prove their experience.