![Lagos44](https://upload.wikimedia.org/wikipedia/commons/9/94/Lagos44.jpg)
The mariner's astrolabe, also called sea astrolabe, was an
inclinometer used to determine the
latitude
In geography, latitude is a 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 the south pole to 90° at the north pol ...
of a ship at sea by measuring the sun's noon altitude (declination) or the
meridian altitude Meridian altitude is a method of celestial navigation to calculate an observer's latitude. It notes the altitude angle of an astronomical object above the horizon at culmination.
Principle
Meridian altitude is the simplest calculation of celestial ...
of a star of known declination. Not an
astrolabe proper, the mariner's astrolabe was rather a graduated circle with an
alidade
An alidade () (archaic forms include alhidade, alhidad, alidad) or a turning board is a device that allows one to sight a distant object and use the line of sight to perform a task. This task can be, for example, to triangulate a scale map on site ...
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 was invented by the Portuguese people, a nation known for its maritime prowess and dominated the sea for multiple centuries. In the sixteenth century, the instrument was also called a ring.
History
![Silver-plated Mariner's Astrolabe](https://upload.wikimedia.org/wikipedia/commons/3/37/Silver-plated_Mariner%27s_Astrolabe.jpg)
Many dates can be found for the appearance of the first Mariner's astrolabes. The earliest date, 1295, is offered by the
Majorcan
Mallorca, or Majorca, is the largest island in the Balearic Islands, which are part of Spain and located in the Mediterranean.
The capital of the island, Palma, is also the capital of the autonomous community of the Balearic Islands. The Balea ...
astronomer
Ramon Llull.
[''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 English Anglican cleric who published several volumes of reports by travellers to foreign countries.
Career
Purchas was born at Thaxted, Essex son of an English yeoman. He graduated from St John's College, Cam ...
claiming that it was adapted for marine navigation by
Martin Behaim
Martin Behaim (6 October 1459 – 29 July 1507), also known as and by various forms of , was a German textile merchant and cartographer. He served John II of Portugal as an adviser in matters of navigation and participated in a voyage to W ...
,
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.
There is strong evidence that the mariner's astrolabe was derived directly from the planispheric
astrolabe, 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
The term Jacob's staff is used to refer to several things, also known as cross-staff, a ballastella, a fore-staff, a ballestilla, or a balestilha. In its most basic form, a Jacob's staff is a stick or pole with length markings; most staffs ar ...
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
The backstaff is a navigational instrument that was used to measure the altitude of a celestial body, in particular the Sun or Moon. When observing the Sun, users kept the Sun to their back (hence the name) and observed the shadow cast by the u ...
. By the late 19th century, mariners began using the
sextant and then
global positioning system
The Global Positioning System (GPS), originally Navstar GPS, is a satellite-based radionavigation system owned by the United States government and operated by the United States Space Force. It is one of the global navigation satellite sy ...
s (GPS) starting in the 1980s.
![Sodre Astrolabe 1503](https://upload.wikimedia.org/wikipedia/commons/9/90/Sodre_Astrolabe_1503.jpg)
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
The Corpus Christi Museum of Science and History is a science and history museum in Corpus Christi, Texas, near Corpus Christi Harbor Bridge
The Corpus Christi Harbor Bridge is a through arch bridge located in Corpus Christi, Texas which carries ...
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 (Cu) and zinc (Zn), in proportions which can be varied to achieve different mechanical, electrical, and chemical properties. It is a substitutional alloy: atoms of the two constituents may replace each other wit ...
. 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
Ivory is a hard, white material from the tusks (traditionally from elephants) and teeth of animals, that consists mainly of dentine, one of the physical structures of teeth and tusks. The chemical structure of the teeth and tusks of mammals i ...
, 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
Limb may refer to:
Science and technology
*Limb (anatomy), an appendage of a human or animal
*Limb, a large or main branch of a tree
*Limb, in astronomy, the curved edge of the apparent disk of a celestial body, e.g. lunar limb
*Limb, in botany, ...
. The sole purpose of the spokes was to support the pivot point for the
alidade
An alidade () (archaic forms include alhidade, alhidad, alidad) or a turning board is a device that allows one to sight a distant object and use the line of sight to perform a task. This task can be, for example, to triangulate a scale map on site ...
. In order to lower the
centre of gravity
In physics, the center of mass of a distribution of mass in space (sometimes referred to as the balance point) is the unique point where the weighted relative position of the distributed mass sums to zero. This is the point to which a force ma ...
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
An alidade () (archaic forms include alhidade, alhidad, alidad) or a turning board is a device that allows one to sight a distant object and use the line of sight to perform a task. This task can be, for example, to triangulate a scale map on site ...
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
![Astrolabe (PSF)](https://upload.wikimedia.org/wikipedia/commons/9/90/Astrolabe_%28PSF%29.png)
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 which Portugal initiated. Portuguese Sailors trained British sailors and sailors from another nations, one of the most famous was
Christopher Columbus
Christopher Columbus
* lij, Cristoffa C(or)ombo
* es, link=no, Cristóbal Colón
* pt, Cristóvão Colombo
* ca, Cristòfor (or )
* la, Christophorus Columbus. (; born between 25 August and 31 October 1451, died 20 May 1506) was a ...
in how to use this instrument and 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 or height (also sometimes known as depth) is a distance measurement, usually in the vertical or "up" direction, between a reference datum and a point or object. The exact definition and reference datum varies according to the context ...
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 lette ...
calculations by the
lunar distance method
Lunar most commonly means "of or relating to the Moon".
Lunar may also refer to:
Arts and entertainment
* ''Lunar'' (series), a series of video games
* "Lunar" (song), by David Guetta
* "Lunar", a song by Priestess from the 2009 album ''Prior t ...
(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
An alidade () (archaic forms include alhidade, alhidad, alidad) or a turning board is a device that allows one to sight a distant object and use the line of sight to perform a task. This task can be, for example, to triangulate a scale map on site ...
, which was not very long.
See also
*
Davis quadrant
The backstaff is a navigational instrument that was used to measure the altitude of a celestial body, in particular the Sun or Moon. When observing the Sun, users kept the Sun to their back (hence the name) and observed the shadow cast by the u ...
*
Cross staff
The term Jacob's staff is used to refer to several things, also known as cross-staff, a ballastella, a fore-staff, a ballestilla, or a balestilha. In its most basic form, a Jacob's staff is a stick or pole with length markings; most staffs ar ...
*
Octant (instrument)
The octant, also called a reflecting quadrant, is a reflecting instrument used in navigation.
Etymology
The name ''octant'' derives from the Latin ''octans'' meaning ''eighth part of a circle'', because the instrument's arc is one eighth of a ...
*
Sextant
*
Age of Discovery
The Age of Discovery (or the Age of Exploration), also known as the early modern period, was a period largely overlapping with the Age of Sail, approximately from the 15th century to the 17th century in European history, during which seafarin ...
*
Exploration
Exploration refers to the historical practice of discovering remote lands. It is studied by geographers and historians.
Two major eras of exploration occurred in human history: one of convergence, and one of divergence. The first, covering most ...
References
{{Reflist, 2
External links
Make your own mariner's astrolabeChamplain's astrolabe at the Canadian Museum of HistoryBelieved to be
Samuel de Champlain's lost astrolabe.
The Mariners' Museum Navigational and Scientific Instrument CollectionThe Mariner's Astrolabe Catalogue
Astronomical instruments
Inclinometers
Navigational equipment
Historical scientific instruments
Celestial navigation