What Is an Astrolabe and How Was It Used in Navigation?
The astrolabe was one of the most important astronomical instruments of the Middle Ages and the Renaissance, and the mariner’s astrolabe was the principal astronomical instrument aboard the ships of the Age of Exploration. To understand the great voyages of the fifteenth and sixteenth centuries, you have to understand what an astrolabe was and how it was used. The standard scholarly references are 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); Silvio Bedini’s work on early modern scientific instruments is the indispensable companion for the physical artifacts.
The mariner’s astrolabe was not an invention from nothing. It was a stripped-down version of the much older planispheric astrolabe, which had been developed in the Islamic world and transmitted to Europe during the Middle Ages. The European contribution was the simplification, the weight, and the integration of the instrument into a shipboard suite with tables of declination. The result was a rugged, single-purpose tool that could measure the altitude of the sun or a star to within a degree or two, and from that observation a navigator could determine his latitude.
The Astrolabe in General
The astrolabe is an ancient instrument with a long history. Its earliest form, known as the sphera or armillary sphere, was a model of the celestial sphere with rings representing the principal circles of the heavens. From this, Greek astronomers developed the planispheric astrolabe, a flat disk on which the celestial sphere was projected. The planispheric astrolabe became the standard astronomical instrument of the Islamic world and of medieval Europe, used for timekeeping, surveying, and astronomical calculation. For a fuller treatment of the instrument given Age of Exploration navigation, see our page on navigational instruments. The astrolabe was also closely connected to the development of European cartography and maps, since the same mathematical principles that allowed astrolabes to project the celestial sphere also allowed cartographers to project the Earth.
The Planispheric Astrolabe
A planispheric astrolabe was a complex instrument, typically about 15 to 30 centimeters in diameter, made of brass or bronze. It consisted of a main body called the mater, a rotating star map called the rete, one or more interchangeable plates engraved with coordinate lines for different latitudes, and a sighting bar called the alidade. The astrolabe was used to measure the altitude of a celestial body above the horizon, to tell time from the position of the sun or stars, to determine the direction of Mecca, and to solve various astronomical and surveying problems. The planispheric astrolabe was a complex and beautiful instrument, and surviving examples are prized by museums.
The Mariner’s Astrolabe
The mariner’s astrolabe was a much simpler and heavier version of the planispheric astrolabe, designed for use at sea. It typically consisted of a single thick brass or bronze disk, weighing several kilograms, with a graduated scale around the rim and a rotating alidade pivoted at the center. A ring at the top allowed the instrument to be suspended, hanging vertically under gravity. The mariner’s astrolabe had no rete, no interchangeable plates, and no astronomical tables; it was a stripped-down tool for one specific purpose: measuring the altitude of the sun or a star above the horizon. The mariner’s astrolabe is one of the iconic instruments of the era, and it appears in museum collections from Lisbon to London.
The oldest surviving mariner’s astrolabe is the so-called Sodré astrolabe, recovered from the wreck of a Portuguese vessel lost in 1503 off the coast of Al Hallaniyah in the Omani-administered island group of the same name. Scholars date the instrument to around 1480 on stylistic grounds, and it is the earliest known dated example of the type, though other, undated examples survive and the precise origin of the instrument remains debated.
Why the Mariner’s Astrolabe Was Heavy
Unlike the delicate planispheric astrolabe, the mariner’s astrolabe was deliberately heavy. The weight served two purposes: it helped the instrument hang vertically even in a strong wind, and it helped keep the disk stable in the rolling motion of a ship. The instrument was typically used by suspending it from a ring at the top, allowing it to swing freely, and sighting the sun or a star through the pinholes in the alidade. The navigator would read the angle of the alidade on the graduated scale, and this gave the altitude of the celestial body.
How Navigators Used the Astrolabe
To use the mariner’s astrolabe, the navigator suspended the instrument from a ring at the top, allowing it to hang vertically under gravity. He would then rotate the alidade until the sun’s ray, passing through the upper pinhole, fell on the lower pinhole. The angle indicated on the graduated scale was the sun’s altitude above the horizon. The process required a steady hand and a clear horizon, and it was most practical at midday, when the sun was high and the horizon was clear. At night, the navigator could measure the altitude of the Pole Star by sighting along the alidade, but this was less accurate than daytime observations of the sun.
The Latitude Calculation
Once the navigator had measured the altitude of the sun at noon, he could calculate his latitude. The basic procedure was to look up the sun’s declination for that day — the angle of the sun above or below the celestial equator — in a table such as the Alphonsine Tables or the Tabulae Directionum of Regiomontanus. The latitude was then approximately equal to the declination plus or minus the altitude, depending on whether the sun was on the same side of the zenith as the observer or the opposite side. The procedure was well known to Portuguese and Spanish pilots by the late fifteenth century, and it was the basis of the systematic southward exploration of the African coast.
A Practical Example
Suppose a navigator observed the sun at noon on the day of the equinox, when the sun’s declination is zero. If the sun’s altitude at noon was 60 degrees above the southern horizon, then the latitude was 30 degrees north of the equator. The relationship between the sun’s altitude and the latitude was the foundation of all celestial navigation in the Age of Exploration, and it was a major reason that the Portuguese were able to push so far down the African coast in the fifteenth century. The use of latitude observations in practical navigation is described in our page on portolan charts, dead reckoning, and piloting techniques.
Limitations of the Mariner’s Astrolabe
The mariner’s astrolabe had several practical limitations. Its accuracy depended on a steady, vertical reference, and the rolling of a ship could make the instrument difficult to use. The pinholes in the alidade were small, and the navigator had to look directly at the sun, which was uncomfortable and could be dangerous to the eyes. 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 transition from the astrolabe to later instruments is described in detail on our navigational instruments cluster page.
The Astrolabe in the Voyages of Discovery
The mariner’s astrolabe is particularly associated with Portuguese exploration. Portuguese pilots used it to measure the altitude of the sun and the Pole Star on the African coast, and it was a standard instrument aboard the ships of Bartolomeu Dias, Vasco da Gama, and Pedro Álvares Cabral. It was also used by Spanish pilots in the Caribbean, and the surviving examples of mariner’s astrolabes from the fifteenth and sixteenth centuries are concentrated in Portuguese, Spanish, and English collections. The mariner’s astrolabe is one of the iconic instruments of the era, and it is featured in museum exhibits around the world. For the context of the great voyages in which the instrument was used, see our page on Vasco da Gama and the sea route to India. The work of these pilots also laid the empirical foundation for the cartography and maps of the era.