Navigational Instruments: Compass, Astrolabe, and Sextant

The instruments of the Age of Exploration were not, on the whole, European inventions. The magnetic compass came from China. The astrolabe came from the Islamic world. The cross-staff and the kamal had Islamic and Arab predecessors. What European instrument makers of the fourteenth, fifteenth, and sixteenth centuries contributed was the integration of these inherited tools into a portable, shipboard suite that could be used together to estimate position at sea. The argument of this page: the European genius in this period was not the invention of new instruments but the systematic adoption, simplification, and eventual refinement of instruments from across Eurasia, and the development of the institutional infrastructure — pilot schools, observation tables, printed rutters — that made them usable. David Waters’s The Art of Navigation in England and the Netherlands in the Sixteenth and Seventeenth Centuries (1958) and Eric May’s History of Maritime Navigation (1973) are the indispensable treatments.

The development of navigational instruments was a slow, cumulative process. Between the thirteenth and the eighteenth centuries, European pilots went from having no reliable way to determine direction at sea to having a working set of tools — compass, mariner’s astrolabe, cross-staff, backstaff, quadrant, chip log, half-hour glass — that allowed them to estimate latitude with reasonable accuracy and to keep a running dead reckoning of their position. The longitude problem, the central unsolved question of the era, was not fully addressed until the development of the marine chronometer by John Harrison in the 1760s. The instruments themselves are covered in greater detail on our astrolabe long-tail page.

The Magnetic Compass

The magnetic compass was the foundational instrument of ocean navigation. Its history is unambiguously Chinese. The earliest compasses, developed in China during the Han dynasty, used a magnetized needle or lodestone spoon floating on water to indicate direction. The Chinese text Dream Pool Essays of Shen Kuo (1088) describes the magnetic compass in detail, and Chinese mariners were using wet compasses (a magnetized needle floated on water) for navigation by the eleventh century. The compass reached the Islamic world by the twelfth century and was transmitted to Europe, probably through Italy, by the thirteenth. The European dry compass, in which a magnetized needle was mounted on a pivot inside a wooden bowl, was developed in the fourteenth century.

By the fifteenth century, the European mariner’s compass had been refined into a reliable instrument. A magnetized needle was mounted on a pivot inside a wooden bowl, with a graduated “compass card” attached to the needle so that the whole card turned as the needle aligned with magnetic north. The bowl was mounted in a binnacle or suspended in gimbals to keep it level in rough weather. The navigator could read the ship’s heading off the card by reference to a fixed mark on the inside of the bowl called the “lubber’s line.”

Compass bearings of the period were divided into 32 points of 11.25 degrees each, with half- and quarter-points giving finer resolution. The four cardinal directions — north, east, south, west — were supplemented by intercardinal points (NE, SE, SW, NW), and the 32-point system was the standard for European navigation from the fourteenth to the eighteenth century. In the seventeenth and eighteenth centuries, compasses began to be graduated in degrees (360) rather than compass points, and azimuth compasses with finer graduations became common.

Magnetic Variation

One of the persistent problems with the compass was magnetic variation — the difference between magnetic north and true north. Already in the fifteenth century, navigators were aware that a compass did not point exactly to true north, and that the amount of variation changed with location. Columbus famously noted the variation of the compass during his first Atlantic crossing in 1492, when his needles pointed slightly northwest of north as he approached the Bahamas. The English compass maker Robert Norman wrote about magnetic dip in 1581, and Edmond Halley and other scientists worked on the problem in the seventeenth and eighteenth centuries, with Halley publishing the first magnetic variation chart for the Atlantic in 1701. The accurate mapping of magnetic variation was one of the great practical projects of early modern science, and it was directly tied to the needs of ocean navigation. Alan Gurney’s Compass: A Story of Exploration and Innovation (2004) is a readable treatment.

The Mariner’s Astrolabe

The mariner’s astrolabe was the principal astronomical instrument of the Age of Exploration. It was a simplified, heavy version of the much older astronomical astrolabe, which had been developed in the Islamic world and transmitted to Europe during the Middle Ages. A mariner’s astrolabe typically consisted of a heavy brass or bronze disk, pierced with a graduated scale around the rim, and a rotating alidade (a sighting bar) pivoted at the center. The navigator would suspend the instrument from a ring at the top, allowing it to hang vertically under gravity, and would sight the sun or a star through the pinholes in the alidade. The angle of the alidade on the graduated scale gave the altitude of the celestial body above the horizon.

The oldest known mariner’s astrolabe is the so-called Sodré astrolabe, recovered from the wreck of a Portuguese vessel lost in 1503 off the coast of Omani-administered Al Hallaniyah island. It is the earliest known dated example of the type, although the term “mariner’s astrolabe” was not in use at the time of its construction (probably c. 1480), and the instrument is best understood as a Portuguese adaptation of the Islamic planispheric astrolabe. The mariner’s astrolabe is particularly associated with Portuguese exploration of the African coast. Portuguese pilots used the instrument to measure the altitude of the sun at noon and the Pole Star at night, and from these observations they could calculate their latitude using simple trigonometric tables. The use of the mariner’s astrolabe drove the systematic southward progress of Portuguese exploration along the African coast during the fifteenth century, and it was a standard instrument aboard the ships of Bartolomeu Dias, Vasco da Gama, and Pedro Álvares Cabral.

The mariner’s astrolabe had several practical limitations. Because the instrument was suspended from a ring and depended on gravity to remain vertical, it was hard to use on a moving ship in heavy weather. The pinholes in the alidade were small, and the sun was a difficult object to sight directly. Graduations on instruments of the period were often coarse, with intervals of 5 degrees or more, and a single observation might be accurate to only a degree or two. For these reasons, the mariner’s astrolabe was gradually supplanted by the cross-staff, the backstaff, and eventually the quadrant and the reflecting sextant.

The Cross-Staff

The cross-staff, also known as the balestilha in Portuguese or Jacob’s staff in English, was a simpler and more practical instrument than the mariner’s astrolabe. It consisted of a long wooden staff along which a smaller crossbar could slide. The navigator would place one end of the staff at his eye, slide the crossbar until its ends just touched the horizon and the celestial body, and read the altitude off a scale on the staff. The cross-staff was widely used by navigators of the late fifteenth and sixteenth centuries, and it became the standard instrument for sun-shooting during the great Spanish and Portuguese voyages of discovery.

To use the cross-staff, the navigator held the staff at eye level and looked along it toward the horizon, sliding the crossbar up and down until its lower end just touched the horizon and its upper end just touched the sun or star. The reading on the staff at the lower end of the crossbar gave the altitude of the body. The technique required a steady hand and a clear horizon, and it was most practical when the sun was low in the sky. The Portuguese pilot’s use of the cross-staff, combined with tables of solar declination, drove the success of the voyages down the African coast. The cross-staff is sometimes called the radius astronomicus, and it remained a standard navigation instrument until the late sixteenth century, when the backstaff and the quadrant began to replace it.

The cross-staff required the navigator to look directly at the sun, which could be painful and dangerous to the eyes. The instrument was also limited in the altitudes it could measure: for high altitudes the crossbar had to be very long, making it unwieldy. The discomfort of using the cross-staff, especially in tropical waters, was one of the motivations for the development of the backstaff.

The Backstaff, Quadrant, and Kamal

The backstaff, also called the Davis quadrant after its English inventor John Davis, was developed in the 1590s to measure the sun’s altitude without looking directly at the sun. The navigator looked at the shadow of the sun cast by a vane on a horizontal arc, while a second vane on a vertical arc was adjusted to align with the horizon. The reading on the combined arcs gave the sun’s altitude. The backstaff was easier on the navigator’s eyes than the cross-staff and was the principal sun-shooting instrument of the seventeenth and eighteenth centuries.

The quadrant was a similar instrument, with a graduated arc of 90 degrees and a sighting vane. It could be used with the sun or with the Pole Star, and various forms were used throughout the sixteenth, seventeenth, and eighteenth centuries. Arabic navigators had used a similar instrument for centuries. The kamal, a simple piece of knotted string used by Arab navigators of the Indian Ocean, was another traditional altitude instrument that was sometimes used by European navigators in the Red Sea and the Indian Ocean. The kamal is treated, along with the broader tradition of non-Western navigation, in David Lewis’s We, the Navigators (1972), which remains the standard work on Pacific and Indian Ocean techniques.

The Reflecting Sextant

The reflecting sextant, the most accurate of all early-modern altitude instruments, was developed in the eighteenth century and was not part of the Age of Exploration proper. Invented independently by John Hadley in England and Thomas Godfrey in America around 1731, the sextant used a system of mirrors to bring the image of the sun or a star into coincidence with the horizon, allowing accurate altitude measurements to within a fraction of a minute of arc. Combined with the marine chronometer, developed by John Harrison in the same period, the sextant finally solved the longitude problem and made accurate ocean navigation possible. Dava Sobel’s Longitude (1995) is the readable account of the Harrison story, although the more technical treatment is in Andrewes’s edition of Harrison’s own papers. The sextant represents the culmination of a long technological tradition that began with the compass and the mariner’s astrolabe.

Timekeeping at Sea

The most fundamental problem of ocean navigation throughout the Age of Exploration was the difficulty of measuring time accurately at sea. A spring-driven clock or watch would be disturbed by the motion of the ship, and no such timekeeper could be relied on for the weeks-long voyages of the era. Sailors therefore depended on the half-hour glass — a sealed vessel of sand that ran for thirty minutes — to mark the watches of the day. Eight half-hour glasses made a four-hour watch, and the standard system of watches and bells became the basic rhythm of life on board. Time of day could also be estimated from the position of the sun, but the result was a rough approximation rather than a precise measurement.

The absence of accurate timekeeping was a major limitation on the determination of longitude. Longitude could, in principle, be calculated by comparing the local time at the ship (determined by sun observations) with the time at a reference meridian (such as Greenwich or Paris), and the difference in hours gave the difference in degrees. But without a reliable clock to carry the reference time across the oceans, the method was impractical. It was not until John Harrison’s marine chronometer, developed in the 1730s and 1740s and made practical by the 1760s, that longitude could be measured with anything like precision. The Board of Longitude offered £20,000 for a practical solution; Harrison eventually received the bulk of the prize money in 1773, after decades of test voyages.

Chip Logs and Speed Measurement

To measure a ship’s speed, navigators used a chip log: a wooden board, weighted along its bottom edge and attached to a line with regularly spaced knots. The board was thrown overboard, and the line was allowed to run out for a fixed time interval, measured by a half-minute glass. Counting the number of knots that ran out in that interval gave the ship’s speed in nautical miles per hour — the origin of the term “knot” as a unit of speed. The chip log was developed in the sixteenth century and was the standard method of speed measurement throughout the Age of Exploration and into the nineteenth century. Combined with the compass and the half-hour glass, the chip log was one of the three tools that made dead reckoning possible — a technique explained in detail on our dead reckoning long-tail page.

Logbooks, Rutters, and Pilot Books

In addition to physical instruments, navigators of the Age of Exploration relied on a body of written knowledge: the logbook, in which the master recorded course, speed, weather, and sightings; the rutter (or routier), a sailing direction describing coasts, harbors, currents, and hazards; and the pilot book, a compilation of practical advice for specific regions. The earliest printed rutters appeared in the late fifteenth century, and they circulated widely among European pilots. The English polymath John Dee, the Portuguese navigator Duarte Pacheco Pereira (whose Esmeraldo de situ orbis, c. 1505–1508, is the most important early Portuguese pilot-book), and the Dutch cartographer Lucas Janszoon Waghenaer were among the most important authors of pilot books and rutters. Waghenaer’s Spieghel der Zeevaerdt (1584–1585), translated into English as the Mariner’s Mirror, was one of the most influential pilot books of the era.

The Legacy

The instruments of the Age of Exploration shaped the world in ways that went beyond navigation. The compass enabled global maritime trade. The sextant and the chronometer made possible the great age of European hydrography in the eighteenth and nineteenth centuries, including the voyages of Captain James Cook. The standardization of latitude and longitude, the use of the metric and nautical systems of measure, and the very concept of a navigable global ocean all descend from the technical revolution of the fifteenth and sixteenth centuries. The cartographer and instrument maker were, in this sense, the makers of the modern world, and the Age of Exploration was their workshop.