Navigation Tools

Sand glass. At first, time on a ship was measured by a sand glass. But this was little use on a long voyage, as it could only measure short periods of time. However, a half-minute sand glass was often used with a log line (a rope with knots tied in it at regular intervals) to measure speed. The log line was paid out behind the ship and the speed was calculated by measuring the time between knots as the line went out. A ship’s speed is still given in “knots” today.

Compass. The earth is like a giant magnet. It has two magnetic poles, which lie near the North and South Poles. Therefore a needle of magnetized iron will always point to the magnetic poles if allowed to swing freely. A magnetic compass works according to this principle. Europeans did not develop a magnetic compass until about 1200. Compasses were used on board ship to tell sailors in which direction they were sailing. Early compasses were not very reliable. A compass needle could be affected by other iron objects nearby, such as a ship’s cannon, so voyages often went astray.

Quadrant. The quadrant was probably the first instrument used by navigators to measure the height of a star in order to calculate latitude. It was a quarter-circle of brass, with a plumb line hanging straight down from the point. One of the straight edges had tiny holes at each end. The navigator looked at the star through these holes. The plumb line then showed the height of the star in degrees, which were marked along the curved edge.

Astrolabe. The astrolabe was a device for measuring the height of the Sun at noon. This told the navigator his latitude. Like many other navigational instruments, it was first used by astronomers, people who study the stars. An astrolabe was a disc with degrees marked on a circular scale around the edge and a rotating arm that had a small eyehole at each end. The navigator turned the arm until the sunlight shone through the two eyeholes. The pointer at the end of the arm then indicated the height of the Sun in degrees above the horizon.

Cross-staff. The cross-staff was an instrument used for judging latitude by measuring the height of a star. The navigator lined the cross-staff up with the horizon, then moved the sliding cross-piece until the top was in line with the star. The long arm had a scale on it, which was marked with degrees, and the position of the cross-piece gave the height of the star in degrees above the horizon. The cross-staff was easier to use than an astrolabe, but was no use in daytime because the human eye cannot look directly at the sun. A more complicated version, called a back-staff or English quadrant, which was invented later, solved this problem by allowing the navigator to take a reading with his back to the Sun.

Nocturnal. The nocturnal was invented in about 1550 and was used to tell the time at night. Holding the handle at arm’s length, the navigator looked at the Polar Star through the hole in the centre of the instrument. He then moved the arm until it lined up with two other stars in the Pole Star’s constellation. The arm pointed to the time on a disk in the middle of the device. The nocturnal was accurate to within about 10 minutes.

Portolan chart. The earliest sailors’ maps were called Portolan charts and were drawn on goatskin. The charts showed places and landmarks along the coast and were covered with direction lines and decorative compasses, known as compass “roses”. These early maps were often inaccurate because their makers did not know enough geography. They were also uncertain how to show the curved surface of the Earth on a flat map. Portolan charts were used a great deal by Portuguese explorers in the 16^th century.

Chronometer. The invention of the chronometer in the 18^th century made sea navigation much easier. A chronometer is an accurate clock, which will keep nearly perfect time even when tossed about in a ship at sea for months. Most importantly, it allowed navigators to measure longitude accurately, because it could be set to keep Greenwich time.

Octant. The octant was invented in about 1730. It was an improved version of the quadrant, with two mirrors. By moving the arm, the navigator brought the reflection of the star together with the reflection of the horizon. The arm then indicated the height of the star in degrees on the scale at the bottom.

Sextant. The sextant replaced the quadrant in the late 18^th century and is still used today to measure a star’s altitude. It is fitted with double mirrors and a telescope for greater accuracy and, unlike the quadrant or octant, it can measure angles greater than 90⁰. Early sextants had to be hand held, so ships’ navigators often used them on the shore, rather than on board ship.

Modern navigation. In the 20^th century, methods of navigation have improved enormously. In 1908, the gyroscopic compass was invented. This always points to the true north and is not affected by magnetism. But the biggest breakthrough in navigational equipment was the invention of radio in around 1900. The chronometer, which was so important in the 18^th century, is now unnecessary because time checks are broadcast by radio. Radio also enables ships to communicate with one another. Today, a ship anywhere in the world can also check its exact position by means of a signal from a satellite in orbit.