Early ship captains used the sun and the stars to determine where they were and where they were going. During the day, the sun's location in the sky gave mariners a good idea of which direction they were moving. Longer voyages necessitated night-time navigation. Sailors learned to use the stars, particularly the North Star, to determine the latitude of their ship. The North Star, Polaris, does not move in the night sky. Measuring the length from this star to the horizon told a ship's pilot how far north or south he was.
Some cultures did not have knowledge of nautical astronomy. They used shore sighting birds, such as the pigeon, to detect nearby land masses. This navigational technique was used by some sailors in the Indian Ocean thousands of years ago.
The astrolabe was a disk made from wood or brass that navigators hung vertically on the ship's deck. Around its edges they marked equally-spaced degrees. Holes at either end of a rotating arm were aligned with these degrees and when the sun shone through both holes the navigator knew the exact direction of travel. Sighting known stars through these holes achieved the same effect at night. The rolling seas made it difficult for sailors using the astrolabe to be accurate.
An improvement on the astrolabe was the quadrant. As its name might suggest, its design used just a quarter of a circle, marked with 90 degrees. Ship's pilots attached a plumb line to hang along its face. When the pilot sighted the North Star along the quadrants edges, the line hung down across one of the degrees telling exactly how far the star was from the horizon. This could accurately determine the ship's latitude.