
The mariner's astrolabe, also called sea astrolabe, was an
inclinometer
An inclinometer or clinometer is an measuring instrument, instrument used for measuring angles of slope, elevation, or depression (geology), depression of an object with respect to gravity's direction. It is also known as a ''tilt indicator'', ' ...
used to determine the
latitude
In geography, latitude is a geographic coordinate system, geographic coordinate that specifies the north-south position of a point on the surface of the Earth or another celestial body. Latitude is given as an angle that ranges from −90° at t ...
of a ship at sea by measuring the sun's noon altitude (declination) or the
meridian altitude of a star of known declination. Not an
astrolabe
An astrolabe (; ; ) is an astronomy, astronomical list of astronomical instruments, instrument dating to ancient times. It serves as a star chart and Model#Physical model, physical model of the visible celestial sphere, half-dome of the sky. It ...
proper, the mariner's astrolabe was rather a graduated circle with an
alidade used to measure vertical angles. They were designed to allow for their use on boats in rough water and/or in heavy winds, which astrolabes are ill-equipped to handle. It's notable for its role during the
Age of Exploration where
Portuguese and
Spanish navigators used it to great success. In the sixteenth century, the instrument was also called a ring.
History

Many dates can be found for the appearance of the first Mariner's astrolabes. The earliest date, 1295, is offered by the
Majorcan astronomer
Ramon Llull
Ramon Llull (; ; – 1316), sometimes anglicized as ''Raymond Lully'', was a philosopher, theologian, poet, missionary, Christian apologist and former knight from the Kingdom of Majorca.
He invented a philosophical system known as the ''Art ...
.
[''The Oxford Companion to Ships and the Sea'', Peter Kemp ed., 1976 ] Later dates center around the late 15th century, with
Samuel Purchas
Samuel Purchas ( – 1626) was an England, English Anglican cleric who published several volumes of reports by travellers to foreign countries.
Career
Purchas was born at Thaxted, Essex, England, Essex, son of a yeoman. He graduated from St J ...
claiming that it was adapted for marine navigation by
Martin Behaim,
a mariner not considered a trustworthy source by some historians. In any event, the instrument was certainly known by the end of the 15th century. Nevertheless, the creation and perfecting of the mariner's astrolabe is attributed to Portuguese navigators during the beginning of
Portuguese discoveries.
The earliest known description of how to make and use a sea astrolabe comes from
Martín Cortés de Albacar’s ''Arte de Navegar'' published in 1551,
[Swanick, Lois Ann. ''An Analysis Of Navigational Instruments In The Age Of Exploration: 15th Century To Mid-17th century'', MA Thesis, Texas A&M University, December 2005] although the basic principle is the same as that of the
archipendulum used in constructing the
Egyptian pyramids
The Egyptian pyramids are ancient masonry structures located in Egypt. Most were built as tombs for the pharaohs and their consorts during the Old Kingdom of Egypt, Old and Middle Kingdom of Egypt, Middle Kingdom periods. At least 138 identi ...
.
There is strong evidence that the mariner's astrolabe was derived directly from the planispheric
astrolabe
An astrolabe (; ; ) is an astronomy, astronomical list of astronomical instruments, instrument dating to ancient times. It serves as a star chart and Model#Physical model, physical model of the visible celestial sphere, half-dome of the sky. It ...
, as the earliest examples retain some of the markings (e.g. ''umbra recta'' and ''umbra versa'') of the prior device without having the same components.
[Daumas, Maurice, ''Scientific Instruments of the Seventeenth and Eighteenth Centuries and Their Makers'', Portman Books, London 1989 ]
The mariner's astrolabe would have replaced or complemented instruments such as the
cross staff or
quadrant as a navigator's instrument. The mariner's astrolabe was used until the middle or, at the latest, the end of the 17th century.
It was replaced by more accurate and easier-to-use instruments such as the
Davis quadrant. By the late 18th century, mariners began using the
sextant and then LORAN C, SatNav/Transit, and then
global positioning system
The Global Positioning System (GPS) is a satellite-based hyperbolic navigation system owned by the United States Space Force and operated by Mission Delta 31. It is one of the global navigation satellite systems (GNSS) that provide ge ...
s (GPS) starting in the 1980s.

Although their heavy brass construction permits their longevity in marine environments,
mariner's astrolabes are very rare today. In 2017, only 108 were known to exist.
The biggest collection remains in museums in Portugal.
The
Corpus Christi Museum of Science and History in Texas, United States, contains a mariner's astrolabe with a confirmed date of 1554, salvaged from the wreck of the ''
San Esteban''.
A disk-type astrolabe dated between 1496 and 1501, now the world's oldest, was discovered in 2014 by the marine scientist
David L. Mearns on the wreck site of a Portuguese nau ''
Esmeralda,'' which sank off the coast of Oman in 1503.
The astrolabe was confirmed by laser scanning, which revealed a series of 18 gradations marking 5-degree intervals, and has been named the Sodré astrolabe. It has been proposed that the Sodré astrolabe is a transitional instrument between the classic planispheric astrolabe from which the first mariner's astrolabes made of brass were adapted and the open‐wheel type astrolabe that came into use sometime before 1517.
Construction
Mariner's astrolabes were made of
brass
Brass is an alloy of copper and zinc, in proportions which can be varied to achieve different colours and mechanical, electrical, acoustic and chemical properties, but copper typically has the larger proportion, generally copper and zinc. I ...
. Since weight was advantageous when using the instrument on the heaving deck of a ship or in high winds, other materials, such as wood or
ivory, were not desirable though some wood sea astrolabes were made. Early sea astrolabes were made from sheets of brass. Due to their light weight, they tended to perform poorly at sea. Heavier cast brass frames began to be made in the mid-sixteenth century and were considerably better.
[Waters, David Watkin, ''The Art of Navigation in England in Elizabethan and Early Stuart Times'', Yale University Press, 1958] As the accuracy of the instrument is related to the radius of the divided circle, these were made as large as practical.
Since the large plate form of the
planispheric astrolabe makes it sensitive to the wind, the mariner's astrolabe is made with a frame form. The openings in the frame allow wind to pass through, inducing less motion in the instrument.
The essential function of the device was to measure angles. Thus the instrument featured a ring graduated in degrees. Early instruments were only
graduated for 90°; later instruments were graduated for the full 360° circle around the
limb. The sole purpose of the spokes was to support the pivot point for the
alidade. In order to lower the
centre of gravity of the device and thus increase its
period of motion as a means of stabilizing it, extra brass was usually added to the bottom of the instrument inside the ring. This is clearly evident in the lower left instrument seen in the photograph above.
The
alidade was free to rotate about a pin through the centre of the instrument. The vanes of the alidade were either slotted or pierced with a hole to allow the user to align the alidade.
The astrolabe had a ring attached to the top of the instrument to allow it to hang vertically.
Usage
In order to use the astrolabe, the navigator would hold the instrument by the ring at the top. This caused the instrument to remain in a vertical plane. The navigator would then align the plane of the astrolabe to the direction of the object of interest. The alidade was aligned to point at the object and the altitude was read.
If observing a dim object such as a star, the navigator would observe the object directly through the alidade. If observing the Sun, it was both safer and easier to allow the shadow of one of the alidade's vanes to be cast onto the opposite vane.
It played a key part of the
Age of Discovery
The Age of Discovery (), also known as the Age of Exploration, was part of the early modern period and overlapped with the Age of Sail. It was a period from approximately the 15th to the 17th century, during which Seamanship, seafarers fro ...
, which Portugal initiated. Portuguese sailors trained British and other sailors – one of the most famous was the Italian-born
Christopher Columbus
Christopher Columbus (; between 25 August and 31 October 1451 – 20 May 1506) was an Italians, Italian explorer and navigator from the Republic of Genoa who completed Voyages of Christopher Columbus, four Spanish-based voyages across the At ...
– in how to use this instrument, which became the key to the maritime growth and conquest of the other European Colonial Empires.
Limitations
The mariner's astrolabe needed to be suspended vertically in order to measure the
altitude
Altitude is a distance measurement, usually in the vertical or "up" direction, between a reference datum (geodesy), datum and a point or object. The exact definition and reference datum varies according to the context (e.g., aviation, geometr ...
of the celestial object. This meant it could not be used easily on the deck in windy conditions. It could not easily be used to measure the angle between two objects, which was necessary for
longitude
Longitude (, ) is a geographic coordinate that specifies the east- west position of a point on the surface of the Earth, or another celestial body. It is an angular measurement, usually expressed in degrees and denoted by the Greek lett ...
calculations by the
lunar distance method (though that technique was not used when the instrument was developed). Another limitation was that the instrument's angular accuracy was directly proportional to the length of the
alidade, which was not very long.
See also
*
Octant (instrument)
*
Exploration
Exploration is the process of exploring, an activity which has some Expectation (epistemic), expectation of Discovery (observation), discovery. Organised exploration is largely a human activity, but exploratory activity is common to most organis ...
References
{{Reflist, 30em
External links
Make your own mariner's astrolabe(archived 20 August 2007)
Champlain's astrolabe at the Canadian Museum of History– believed to be
Samuel de Champlain
Samuel de Champlain (; 13 August 1574#Fichier]For a detailed analysis of his baptismal record, see #Ritch, RitchThe baptism act does not contain information about the age of Samuel, neither his birth date nor his place of birth. – 25 December ...
's lost astrolabe.
The Mariners' Museum Navigational and Scientific Instrument Collection(archived 11 December 2012)
The Mariner's Astrolabe Catalogue
Astronomical instruments
Inclinometers
Navigational equipment
Historical scientific instruments
Celestial navigation