Astrolabe For Ship Navigation

[Adabella Frierdich]

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]

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

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]

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 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.

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.

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.

The astrolabe was well known in the Islamic world during the Middle Ages and some of the great Muslim scientists made further improvements. In Europe they were highly prized tools and knowing how to use one was regarded as a bit of a status symbol. The English writer Geoffrey Chaucer, more famous for his Canturbury Tales, wrote A Treatise on the Astrolabe in 1391 which explained how to use the instrument and was possibly written for his son or godson.

The mariner's astrolabe was a simplified version of an instrument originally developed by Arab astronomers for measuring the height of heavenly bodies above the horizon and came into use in navigation by about 1470. In order to keep it steady when used on board ship, the mariner's version was heavier and had parts of the disc cut away to reduce wind resistance.

The instrument was used to help determine the ship's latitude from the height of the Pole Star or of the sun. At night, the Pole Star was sighted directly through small pinholes in the two vanes mounted on the pivoting alidade or rule. The altitude in degrees was then read off from the scale on the outer edge of the instrument. To measure the Sun's position during the day, the astrolabe was held below the waist and the alidade was adjusted so that a beam of sunlight passed through the top pinhole onto the bottom one.

A sextant can measure an angle on any plane, and works by a principle of double reflection. It is also far more accurate and can be used for a range of purposes including navigation (finding latitude, longitude, local time). An astrolabe can only measure angles in a vertical plane and was principally used for latitude-finding, although you can also use it for purposes such as finding the height of something.

This example was found in 1845 under a rock on Valentia Island, close to the point off southern Ireland where three ships of the Spanish Armada were wrecked in 1588. The throne is low and moulded. The mater has been made from a single casting, cut out in a wheel shape with a greater weight left in the lower half to help the instrument hang vertically.

The face of the mater is engraved with circles and decorative lines but there are no numbers on the scale, suggesting that the instrument was never completed - perhaps it was one of a number of pieces of unfinished equipment hurried aboard a Spanish vessel in 1588. An alidade is fitted to the face of the instrument and held in place with a pin through its centre, about which it can rotate.

By the dawn of the sixteenth century, the ancient art of navigation had begun to develop rapidly in response to oceanic explorers who needed to find their positions without landmarks, to determine the locations of their discoveries, and to establish routes between the new-found lands and home. Although the relationship of certain heavenly bodies to time of day and terrestrial directions had been known since ancient times, the first two decades of the sixteenth century saw the rigorous application of astronomy and mathematics to navigation. The new learning met the New World.

Navigation is based largely on the spherical coordinates latitude -angular distance north or south of the equator - and longitude - angular distance east or west of a generally accepted reference location, such as the Greenwich Observatory. Finding longitude requires comparing local time, measured by a heavenly body, with the local time at a reference location, kept by a clock. Mechanical time-pieces existed in the Elizabethan era, but until the late eighteenth century they had to be corrected frequently by sun sightings and were therefore almost useless aboard ship. Measuring latitude, on the other hand, does not require an accurate time-piece. Refinement of instruments enabled sixteenth-century mariners to determine latitude with reasonable accuracy. Latitude was therefore extremely important to Elizabethan navigation.

Unable to use the latitude-longitude system to the fullest, sixteenth-century navigators supplemented latitude with a rho-theta (distance-and-bearing) system - dead (from deduced) reckoning. Beginning at a known or assumed position, the navigator measured, as best he could, the heading and speed of the ship, the speeds of the ocean currents and the leeward (downwind) drift of the ship, and the time spent on each heading. From this information he could compute the course he had made and the distance he had covered. Dead reckoning, through educated guesswork, is often very accurate. It is still practiced on ships and aircraft, and it lies at the heart of modern doppler and inertial navigational equipment. Errors tend to accumulate in dead reckoning, so its accuracy depends in part on the length of the voyage and the ability of the navigator to use latitude and other information to limit error. But above all else, dead reckoning depends on reliable instruments.

The celestial globe was a mounted sphere depicting the heavens instead of the earth. While many were designed to grace private libraries, some were used as navigational instruments. With the introduction by Gerardus Mercator, in 1569, of practical, affordable sea charts, on which were shown parallels of latitude and meridians of longitude, the costly and delicate celestial globe gradually fell out of use.

The astrolabe was used to determine latitude by measuring the angle between the horizon and Polaris, also called the North Star, the Pole Star, or Stella Maris (Star of the Sea). Polaris was the preferred star for measuring latitude because it is less than one degree from the north celestial pole (the point in the heavens directly above the geographic north pole).

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