The measurement that mattered
The angle of a star or the sun above the horizon is a direct clue to latitude. At noon, the sun’s height, combined with its declination from a table for the day, gives the latitude. In the northern hemisphere, the height of the Pole Star above the horizon is close to the latitude, with a small correction because the star does not sit exactly at the celestial pole. The challenge for a mariner was to take this measurement on a deck that heaved and rolled, with a horizon that could be hazy and with an instrument he could carry and use with one or two hands.
From scholar’s astrolabe to sailor’s ring
The astrolabe is an older instrument, developed in the ancient and medieval worlds and perfected by Islamic astronomers, which models the sky on a flat plate and can be used to solve many problems in astronomy and timekeeping. It is intricate, and it is not well suited to a wet and moving deck.
The mariner’s astrolabe, also called the sea astrolabe, is a much simpler relative. It is typically a heavy ring of brass or bronze, graduated in degrees around part of its edge, with a rotating sighting bar, called an alidade, pivoted at the centre. The instrument was hung from a thumb ring or a cord so that it hung vertically, and the observer turned the alidade until the sun’s rays passed through the sight holes or until the star could be seen along the bar, then read the angle on the scale. The weight kept the instrument steady, and the open frame offered little surface for the wind.
The use of the mariner’s astrolabe is closely associated with Portuguese voyages in the 15th and early 16th centuries, when ships went down the western coast of Africa and later around the Cape route to India. The details of its origin are debated: it is clear that Iberian sailors and scholars drew on the earlier instruments and astronomical learning of the Mediterranean world, including contributions from Jewish and Islamic scholars. Surviving examples come mostly from the 16th and 17th centuries and have been found largely in shipwrecks and from archaeological contexts, which suggests how common they were and how easily they were lost.
The quadrant
The mariner’s quadrant is a quarter of a circle, with a scale of ninety degrees along the curved edge, two sights along one straight edge and a plumb line hanging from the apex. The observer sighted the Pole Star along the edge and read where the plumb line crossed the scale. The quadrant was lighter and cheaper to make than the astrolabe, and it was in use on ships by the 15th century. It suffered in wind, as the plumb line swung, and was probably more difficult to use at sea than the astrolabe, although both were hard at times.
Tables and rules
An instrument alone did not give latitude. The pilot needed rules and tables, which were set out in manuals, such as the Portuguese regimentos of the early 16th century. They gave the corrections for the Pole Star, according to the position of the stars called the guards of the Little Bear, and tables of the sun’s declination for each day of the year. These tables were derived from astronomical work such as that of Abraham Zacuto, a Jewish astronomer whose almanac, produced in the 1490s, was reportedly of value to Portuguese navigators. Pilots were trained in these methods, and ship’s masters and pilots could be examined in their use.
Latitude sailing
The result was a technique called latitude sailing, or running down the latitude. A captain heading to a known place would first sail to its latitude, then follow that parallel east or west until land was sighted. It worked best where the destination lay on a long coast or a known island group, and it avoided the problem of longitude. It also had risks: a ship following a parallel could miss a small island, or strike a reef at night.
The same method fitted the great trading routes. The caravel and carrack carried Portuguese captains along the African coast with the aid of these instruments, and charts began to show scales of latitude. The Cape route to the Indies depended on a pilot’s ability to judge his latitude as the Pole Star sank below the horizon in the southern hemisphere, after which the sun became the main guide.
Accuracy and difficulty
Even with good tables, readings on a ship were uncertain. The horizon was not always sharp, the ship moved and the instruments were rough. Many pilots took sights ashore where they could, since a measurement on land was considerably better than one taken on a deck. A one-degree error in latitude corresponds to about sixty nautical miles, so careful readings mattered. Historians believe that errors of a degree or more were possible at sea, though good observers could do better in calm weather and on land.
Other instruments
Pilots in other regions used their own tools. Arab and Indian Ocean navigators used the kamal, a rectangular card with a knotted cord held between the teeth, whose knots corresponded to the heights of the Pole Star at various ports; accounts of Arab sea pilots and of Red Sea navigation describe the practice. Europeans soon developed the cross-staff, a long rod with a sliding crosspiece. In 1594 the English navigator John Davis described the backstaff, which let the observer keep his back to the sun and read the sun’s shadow, avoiding the glare of looking at it directly.
These devices were eventually replaced by reflecting instruments in the 18th century, which allowed far more accurate observations and are described in the account of the sextant and celestial navigation. Yet for roughly two centuries after the first Portuguese explorations, the astrolabe, the quadrant and their relatives provided a means of fixing latitude that supported voyages across the oceans of the world.