The mariner's astrolabe: derivation and usage

Derivation. Of the various instruments European navigators took on board their ships to measure celestial altitudes, the most iconic was the so-called mariner’s astrolabe. It is in essence a simplified and modified version of the plane astrolabe in several respects. Only the most basic elements of a plane astrolabe were retained: the sighting tool (alidade) together with the surrounding degree scale. The plane disc of a conventional astrolabe was replace by a heavy metal ring, perforated to minimize the effect of the wind on the instrument; and further metal was sometimes added at the bottom to help stabilize it while taking measurement on the high seas.

Usage. To measure the altitude of an object with the astrolabe, the navigator would hold the instrument by the ring at the top, thereby aligning the horizontal line with the horizon. The navigator could then observe a dim object (such as the Pole Star) directly through the two pinholes in two pinnules or vanes on the alidade (see illustration below). 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.

Left-hand image. A small mariner’s astrolabe, inscribed with the date 1603, discovered by a farm boy near Green Lake, Ontario, in 1867. It has pride of place within the Musée Canadien de l’Histoire in Gatineau, Quebec, thanks to the speculation that it was dropped in May 1613 when the French explorer, Samuel de Champlain, was forced to portage during difficult terrain during his pioneering voyage up the Ottawa River. The sceptics’ view is that it was more probably owned by the Jesuit missionaries who mapped this region some time later.

Right-hand image: Marine astrolabe SJB III found on the shipwreck site of the Portuguese merchant ship Nossa Senhora dos Mártires, lost in 1606 at the Tagus mouth, Lisbon, on a return trip from India. Source: Filipe Castro, James Jobling, and Nicholas Budsberg, Marine Astrolabes Catalogue (2018), p. 4.

Credits: Philipp Nothaft (May 2019) and Howard Hotson (May 2021)