The echo sounder , which is found on almost all seagoing ships, indicates the depth of water by measuring the time interval between the emission of a sonic or ultrasonic signal and the return of its echo from the bottom. Buoys and land based navigation markers are used by qualified pilots who go out and bring the heavy laden cargo ships safely through the Gage Roads into the Fremantle Harbour. Piloting is the most exacting form of navigation because it involves the movement of ships potentially dangerous conditions, especially when there are only poor charts of the area. Any boaties who go to Rottnest Island must be very sure to follow the markers to gain safe entry through the reefs into the many beautiful bays, most of which have shallow reefs. In this classic method, used most commonly in the open sea, the navigator uses celestial bodies that have been identified and grouped into constellations since ancient times.
Celestial navigation makes possible voyages across thousands of miles of unmarked water.
However, clouds, fog, rain, snow, mist, or haze, may prevent the essential sightings of celestial bodies. For celestial navigation the navigator uses a sextant and a chronometer. The astrolabe is an instrument used for measuring the positions of heavenly bodies. It consists of a circle or section of a circle, marked off in degrees, with a movable arm pivoted at the center of the circle. Until the sextant was invented during the 18th century, smaller types of astrolabes were the main instruments used by navigators.