Mariner's astrolabe

Mariner's astrolabe

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The mariner's astrolabe, also called sea astrolabe, was an inclinometer
Inclinometer
An inclinometer or clinometer is an instrument for measuring angles of slope , elevation or depression of an object with respect to gravity...

 used to determine the latitude
Latitude
In geography, the latitude of a location on the Earth is the angular distance of that location south or north of the Equator. The latitude is an angle, and is usually measured in degrees . The equator has a latitude of 0°, the North pole has a latitude of 90° north , and the South pole has a...

 of a ship at sea by measuring the sun's noon altitude (declination) or the meridian altitude
Meridian altitude
Meridian Altitude is an astronomical navigation method of calculating an observer's latitude.- Principle :This is the simplest calculation of astronomical navigation and is when an observer determines his latitude by measuring the altitude of a heavenly object at the time of its meridian passage....

 of a star of known declination. Not an astrolabe
Astrolabe
An astrolabe is an elaborate inclinometer, historically used by astronomers, navigators, and astrologers. Its many uses include locating and predicting the positions of the Sun, Moon, planets, and stars, determining local time given local latitude and longitude, surveying, triangulation, and to...

 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. 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
Kingdom of Majorca
The Kingdom of Majorca was founded by James I of Aragon, also known as James The Conqueror. After the death of his first-born son Alfonso, a will was written in 1262 which created the kingdom in order to cede it to his son James...

 astronomer Ramon Llull
Ramon Llull
Ramon Llull was a Majorcan writer and philosopher, logician and tertiary Franciscan. He wrote the first major work of Catalan literature. Recently-surfaced manuscripts show him to have anticipated by several centuries prominent work on elections theory...

. Later dates centre around the late 15th century, with Samuel Purchas
Samuel Purchas
Samuel Purchas , was an English travel writer, a near-contemporary of Richard Hakluyt.Purchas was born at Thaxted, Essex, and graduated at St John's College, Cambridge, in 1600; later he became a B.D., and with this degree was admitted at Oxford in 1615. In 1604 he was presented by James I to the...

 claiming that it was adapted for marine navigation by Martin Behaim
Martin Behaim
Martin Behaim , was a German mariner, artist, cosmographer, astronomer, philosopher, geographer and explorer in service to the King of Portugal.-Biography:The Behaim family had immigrated to Nuremberg because of religious persecution around...

, 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. The earliest known description of how to make and use a sea astrolabe comes from Martín Cortés de Albacar
Martín Cortés de Albacar
Martín Cortés de Albacar was a Spanish cosmographer. In 1551 he published the standard navigitional textbook Arte de navigar Cortés was born in Bujaraloz, province of Zaragoza, Aragon...

’s Arte de Navegar published in 1551, although the basic principle is the same as that of the archipendulum
Archipendulum
The archipendulum is an ancient ancestor of the spirit level and astrolabe, and was used to check whether a line was horizontal or at a desired inclination. It consisted of a handheld A-shaped construction with a plumbline suspended from the top vertex...

 used in constructing the Egyptian pyramids
Egyptian pyramids
The Egyptian pyramids are ancient pyramid-shaped masonry structures located in Egypt.There are 138 pyramids discovered in Egypt as of 2008. Most were built as tombs for the country's Pharaohs and their consorts during the Old and Middle Kingdom periods.The earliest known Egyptian pyramids are found...

.

There is evidence that the mariner's astrolabe was derived directly from the planispheric astrolabe
Astrolabe
An astrolabe is an elaborate inclinometer, historically used by astronomers, navigators, and astrologers. Its many uses include locating and predicting the positions of the Sun, Moon, planets, and stars, determining local time given local latitude and longitude, surveying, triangulation, and to...

, 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.

The mariner's astrolabe would have replaced or complemented instruments such as the cross staff or quadrant
Quadrant (instrument)
A quadrant is an instrument that is used to measure angles up to 90°. It was originally proposed by Ptolemy as a better kind of astrolabe. Several different variations of the instrument were later produced by medieval Muslim astronomers.-Types of quadrants:...

 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.

Mariner's astrolabes are very rare today. In 1979, only 35 were known to exist, while 65 were known by 1988. Fortunately, as more marine archaeology is done, more mariner's astrolabes will likely be found. Their heavy brass construction permits their longevity in marine environments.

Construction


Mariner's astrolabes were made of brass
Brass
Brass is an alloy of copper and zinc; the proportions of zinc and copper can be varied to create a range of brasses with varying properties.In comparison, bronze is principally an alloy of copper and tin...

. 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
Ivory is a term for dentine, which constitutes the bulk of the teeth and tusks of animals, when used as a material for art or manufacturing. Ivory has been important since ancient times for making a range of items, from ivory carvings to false teeth, fans, dominoes, joint tubes, piano keys and...

, 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. 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
Astrolabe
An astrolabe is an elaborate inclinometer, historically used by astronomers, navigators, and astrologers. Its many uses include locating and predicting the positions of the Sun, Moon, planets, and stars, determining local time given local latitude and longitude, surveying, triangulation, and to...

 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
Graduation (instrument)
-Linear graduation:Linear graduation of a scale occurs on a straight instrument. The graduation can identify linear measures, such as inches or millimetres on a rule. They can also be non-linear such as logarithmic or other transcendental scales....

 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
Frequency
Frequency is the number of occurrences of a repeating event per unit time. It is also referred to as temporal frequency.The period is the duration of one cycle in a repeating event, so the period is the reciprocal of the frequency...

 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. He would 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 off the outer degree scale.

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.

Limitations


The mariner's astrolabe needed to be suspended vertically in order to measure the altitude
Celestial coordinate system
In astronomy, a celestial coordinate system is a coordinate system for mapping positions on the celestial sphere.There are different celestial coordinate systems each using a system of spherical coordinates projected on the celestial sphere, in analogy to the geographic coordinate system used on...

 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
Longitude is a geographic coordinate that specifies the east-west position of a point on the Earth's surface. It is an angular measurement, usually expressed in degrees, minutes and seconds, and denoted by the Greek letter lambda ....

 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

  • Astrolabe
    Astrolabe
    An astrolabe is an elaborate inclinometer, historically used by astronomers, navigators, and astrologers. Its many uses include locating and predicting the positions of the Sun, Moon, planets, and stars, determining local time given local latitude and longitude, surveying, triangulation, and to...

  • Davis quadrant
  • Cross staff
  • Octant (instrument)
    Octant (instrument)
    The octant, also called reflecting quadrant, is a measuring instrument used primarily in navigation. It is a type of reflecting instrument.-Etymology:...

  • Sextant
    Sextant
    A sextant is an instrument used to measure the angle between any two visible objects. Its primary use is to determine the angle between a celestial object and the horizon which is known as the altitude. Making this measurement is known as sighting the object, shooting the object, or taking a sight...


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