Mariner's astrolabe

Source: Wikipedia, the free encyclopedia.

Three Mariner's Astrolabes in the Museum of the Forte da Ponta da Bandeira; Lagos, Portugal

The mariner's astrolabe, also called sea astrolabe, was an inclinometer used to determine the latitude 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 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,[1] which astrolabes are ill-equipped to handle. It was invented by the Portuguese people, a nation known for its maritime prowess that dominated the sea for multiple centuries. In the sixteenth century, the instrument was also called a ring.[2]

History

Mariner's astrolabe from circa 1600.
Mariners' Museum

Many dates can be found for the appearance of the first Mariner's astrolabes. The earliest date, 1295, is offered by the

Portuguese discoveries.[4][5] The earliest known description of how to make and use a sea astrolabe comes from Martín Cortés de Albacar’s[6] Arte de Navegar published in 1551,[7] 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.[8]

The mariner's astrolabe would have replaced or complemented instruments such as the

global positioning systems
(GPS) starting in the 1980s.

Oldest mariner's astrolabe in National Museum of Oman, Muscat[9]

Although their heavy brass construction permits their longevity in marine environments,[7] mariner's astrolabes are very rare today. In 2017, only 108 were known to exist.[10][11] The biggest collection remains in museums in Portugal.[5] 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.[12]

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.[9][11] 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.[13]

Construction

Mariner's astrolabes were made of brass. 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.[2] 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

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

An astrolabe being aligned to a star in the sky

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 and other sailors – one of the most famous was the Italian-born Christopher Columbus – 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

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

References

  1. ^ "What is a mariner's astrolabe?".
  2. ^ a b Waters, David Watkin, The Art of Navigation in England in Elizabethan and Early Stuart Times, Yale University Press, 1958
  3. ^
  4. .
  5. ^ a b "Archived copy". Archived from the original on 22 November 2012. Retrieved 2 April 2013.{{cite web}}: CS1 maint: archived copy as title (link)
  6. ^ See es:Martín Cortés de Albacar for the Spanish Wikipedia biography
  7. ^ a b 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
  8. ^
  9. ^ .
  10. ^ a b Morelle, Rebecca (24 October 2017). "Astrolabe: Shipwreck find 'earliest navigation tool'". BBC News. BBC.com. Retrieved 24 October 2017.
  11. ^ "Astrolabe". Texasbeyondhistory.net. Retrieved 14 July 2014.
  12. .

External links