Mariner's astrolabe

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. Later dates center around the late 15th century, with Samuel Purchas claiming that it was adapted for marine navigation by Martin Behaim, 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, 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. 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. Although their heavy brass construction permits their longevity in marine environments, The biggest collection remains in museums in Portugal. 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. ==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. 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, which Portugal initiated. Portuguese sailors trained British and other sailors – one of the most famous was 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 altitude 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 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==