Guess Guess Guess Solve the Case

When was the captain murdered?\One morning around 9 o'clock, Xiao Wang went for a walk by the sea and was astonished to see a small sailboat tilted on the beach. At that time, it was low tide. The more Xiao Wang thought about it, the stranger it seemed to him, so he walked closer to the sailboat. When he reached the side of the boat, he shouted a few times into the cabin, but no one answered. This made Xiao Wang even more curious. He climbed onto the deck along the anchor rope and looked into the dark cabin from the deck stairs. What appeared before his eyes was the captain lying in a pool of blood, a short sword stuck in his chest, seemingly stabbed to death.\In the captain's hand was a torn old nautical chart, and at the head of his bed lay an extinguished candle standing upright horizontally. Perhaps the captain was killed while looking at the chart with the candle lit. After killing the captain, the murderer blew out the candle, took the nautical chart, and then escaped.\Xiao Wang thought this was a murder case of great importance, so he immediately called the police. After the police arrived, they began looking for clues.\"This boat probably docked here around noon yesterday. The cabin is also very dark during the day, so even reading a chart in the daytime requires a candle. Therefore, the captain might not have been killed at night. But precisely when was the captain attacked?"\The police checked the body while discussing.\"The captain was murdered around 9 o'clock last night," Xiao Wang judged decisively.\Can you guess what Xiao Wang based his bold judgment on?