The officers and captain navigated the ship by the sun and stars, using various instruments to work out the position of the ship or the time, and a lead to check the depth of water to avoid running aground. The other essentials were a compass and a chronometer. They used the nocturnal to tell the time at night by the stars, usually using the Pole Star and the constellation of the Great Bear. The date is set on the outer disc using the pointer. The nocturnal is lined up with the constellations using the index arm, and the time read off the inner disc.