Mariner's Astrolabe (1616)



Mariner's Astrolabe 1616

Signed: Elias Allen fecit 1616.
Used for navigation at sea. Brass. Weight 7.7kg. Diameter 39.6cm , depth1.6cm .

The mariner's astrolabe was a navigational device intended for use primarily at sea. It was a modified version of larger, more cumbersome planispheric astrolabes, of which type the university posses an example made by Humphrey Cole. The Astrolabes in the School of Physics and Astronomy were most likely acquired by James Gregory, Professor of Mathematics, during a trip to London in the summer of 1673. He was authorized by the university to purchase all manner of objects he deemed necessary to establish an observatory in St Andrews. Owing to the large amount of items he bought Gregory found it time consuming and tedious to give detailed descriptions of each instrument he brought back, he choose to categorize them into general types.

The origins of the astrolabe are obscure. The earliest surviving one is an Arab example, introduced to Europe via Spain by the Arabs, which dates from 1026/27. This earliest example is a of the planispheric type which is large, thin and usually composed of solid brass. The lightness and high wind resistance of the planispheric variety made them unsuitable for use at sea. From about 1480 navigators began to carry modified versions we now refer to as `Mariners Astrolabes'. The thin plates were replaced by smaller thicker heavily weighted spoked instruments. This offered less wind resistance and more stability on a vessel at sea, which is subject to conditions that make it difficult to make precise navigational measurements. These devices enabled sailors to travel in an East -West direction away from the sight of land. The navigator would frequently sight on the North star with the movable arm on the astrolabe, the Alidade, and measure the angle of height of the north star above the horizon, thereby determining the ships position. In conjunction with other tools such as the panorganon ( also known as a quadrant) the distance the ship needed to sail before coming to the known latitude of a port could be reliably calculated.

The St Andrews Mariner's Astrolabe is one of only 39 known to exist in the world. It is also one of the largest and heaviest (7.7kg) and the only example known to have originated in England as it is marked `Elias Allen fecit 1616' . Allen was one of the finest mathematical instrument makers of the 17th century and for a time kept a shop in London near Essex Gate, Strand. This astrolabe is also one of the very few to be in operable condition, for several reasons: mariner's astrolabes were very expensive to buy, beyond the means of most seamen, they were made for a comparatively short length of time before they were replaced by more convenient and accurate navigational aides such as the cross-staff, and most were simply lost at sea. The majority of astrolabes in existence today have been recovered from wrecks, or, in one case, washed up on a beach after a storm.

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9 May 1996