Staying in sight of land
For most of maritime history, the safest way to navigate was to keep the coast in view. A skipper in the ancient Mediterranean, the Red Sea or the North Sea relied on remembered landmarks, the colour of the water, the depth beneath the keel and the behaviour of birds and winds. Knowledge was passed from pilot to apprentice by speech, and much of it was never written down.
Where it was written down, the results were sailing directions rather than maps. Ancient Greek and Roman texts known as periploi listed ports, headlands and the distances between them in the order a ship would meet them. The idea survives in the modern pilot book. This kind of coastal pilotage never disappeared; even today, harbour approaches are navigated with great attention to marks, depths and tides.
Reading sea, sky and wind
Some peoples learned to cross open water without written aids at all. The tradition of Polynesian voyaging is the best-known example: navigators memorised the rising and setting points of stars, read swell patterns and watched for signs of land such as cloud shapes and seabirds. Their canoes settled islands across a vast area of the Pacific. The details of how the knowledge was held and taught were largely oral, and modern accounts draw on a mix of ethnography and experimental voyages.
In the Indian Ocean, Arab sea pilots combined star knowledge with a good understanding of the monsoon trade winds, which blow in a predictable seasonal pattern and made long passages between East Africa, Arabia and India practical for centuries. Norse seafarers crossed the North Atlantic in the early medieval period using similar observation and judgement; how exactly they fixed their direction remains debated, since few written sources describe it.
The compass and the chart
A magnetised needle that points roughly north is the single most familiar navigation tool. Knowledge of the lodestone’s direction-finding property appears in Chinese sources, and by roughly the 12th and 13th centuries it was in use on ships in both East Asia and Europe. Its great value was that it worked when the sun and stars were hidden. A fuller account is given in the article on the magnetic compass.
By the late 13th century the Mediterranean had a new kind of document. Portolan charts, drawn on parchment and covered with crisscrossing direction lines, let a pilot lay off a bearing from one port to another. They were designed to be used with a compass and a rough estimate of distance. They did not carry a grid of latitude and longitude at first.
Dead reckoning
Between fixed points, sailors relied on dead reckoning: starting from a known position and adding up the course steered and the distance run. Direction came from the compass. Speed was judged by eye for many centuries, and later measured by streaming a floating chip of wood on a line knotted at intervals and counting the knots paid out in a set time, which is the origin of the word “knot” for a nautical mile per hour. A glass timed the watch, and the results were chalked on a slate and then copied into the log.
Dead reckoning is simple, and it accumulates error: currents, leeway and poor estimates of speed all add up. A navigator therefore tried to correct it whenever a landmark, a sounding or an observation of the sky allowed.
Measuring the heights of stars
Latitude, the distance north or south of the equator, can be found by measuring the height of the Pole Star or of the sun at noon above the horizon. In the age when Portuguese ships worked their way down the African coast, mariners adopted simplified versions of the astronomer’s instruments to do this. The mariner astrolabe and the quadrant are described separately; both were hand-held and had to be used on a moving deck. Later, the cross-staff and the backstaff, and then the sextant and celestial navigation as a routine practice, made the measurement faster and more accurate.
This practice is closely tied to the oceanic voyages of the 15th and 16th centuries. Ships such as the caravel and carrack could now sail far from familiar coasts, and a captain who knew the latitude of his destination could sail to that parallel and then run along it. The method gave no answer on how far east or west the ship was. It was good enough for many routes, though not all, and accounts of the era make it clear that errors of position were common. The broader picture of the navigation instruments of the period belongs to the age of exploration.
Charts, projection and survey
As voyages lengthened, charts needed to work over great distances. A chart in which a straight line of constant compass bearing could be drawn with a ruler made course plotting far easier, and the projection published by Gerardus Mercator in 1569 provided one, although it took a long time to become the standard. At the same time, naval and national survey organisations began to measure coasts and soundings systematically. By the 18th and 19th centuries, expeditions such as those of Captain Cook and later the Beagle survey voyages added greatly to the quality of the charts available.
The longitude problem
Finding longitude, the position east or west, needs knowledge of the time at a fixed reference point compared with local time, because the sun appears to move 15 degrees of longitude each hour. The difficulty was carrying accurate time on a pitching, damp, temperature-varying ship. Two answers emerged in the 18th century: astronomical methods that used the moon’s position, and the marine chronometer. Each has its own article. By the early 19th century, chronometers were widespread on long voyages, and the Cook Pacific voyages are among the best-known early examples of their use.
Lighthouses, lightships and buoys
Navigation is not only a matter of what a ship carries. Ports and coasts have been marked with fires, towers, floating lights and buoys for centuries, so that a navigator can recognise a danger or confirm a position. The story of lighthouse development and of lightship stations runs alongside the story of instruments and charts.
Radio, radar and satellites
In the 20th century, electricity changed the picture. Radio direction finding and time signals arrived early in the century. The gyrocompass, which finds north by the rotation of the Earth rather than magnetism, appeared in the 1900s and 1910s. Echo sounders replaced the lead line for many purposes. Marine radar, developed during and after the Second World War, allowed a watchkeeper to see coasts, ships and buoys in fog or at night. Hyperbolic radio systems such as Decca and Loran provided positions from shore transmitters, and from the 1960s onward satellite systems gradually replaced them. Satellite positioning is now routine, and electronic charts have largely replaced paper on large ships, though many navigators still keep traditional skills and instruments as a backup.
A layered tradition
Looking across this long history, new methods rarely replaced older ones at a stroke. A modern ship’s officer may check a satellite fix against a radar picture, a depth reading and a visual bearing, which is in effect the same cross-checking that a medieval pilot did with a lead line, a compass and a familiar headland.