Phases of the moon
It would be easy to assume that the fisherman had quickly lost track of time out on the sea, with nothing to help him mark the days. How could he know that he had been out on the water that long? Alvarenga says that the reason he didn’t lose track of time was an old trick that his grandfather had taught him as a young boy.
He had learned how to mark the passage of time by watching the phases of the moon, seeing it wax and wane meant that he knew how many months he had been on the boat when he was questioned by the police. They weren’t the only ones who wanted to ask him questions, however.