Replicas and Restorations of Early Instruments by Richard A. Paselk

Richard Paselk, Curator

Mariner's Astrolabes

The mariner's astrolabe was probably first developed as a navigational tool by the Portuguese under Prince Henry the Navigator in the late fifteenth century. It is thought to have derived from the back of the planispheric astrolabe by Prince Henry's astronomers. Initially both large (e.g. 24" diameter wooden) and small (e.g. 6" diameter solid bronze) astrolabes were carried by Portuguese explorers down the coast of Africa for measuring the altitude of the Pole star or the Sun to map their progress. The large astrolabes were hung from a framework and probably used only on land. Neither was an effective instrument abroad ship as evidenced by continual early complaints.

The early (<1500 ce) instruments suffered from two problems: 1) the sight vanes on the alidade were too widely spaced, which gives better angular precision as wanted in the planispheric astrolabe, but makes them hard to align on a tossing ship, and 2) the solid disc is readily tossed by the wind, even when as thick as 3/4"–1". This type of instrument is represented by the "Columbus" astrolabe in this collection. Columbus himself wrote in his diary that he never used his astrolabe as it was too unreliable.

Somewhere in the early 16th century the sight-vanes were brought closer together making the instrument somewhat improved. Instruments with a spoked wheel design are first reported in the early 16th century by an Italian spy (navigational knowledge was closely held by Spain and Portugal at this time). These astrolabes are heavy, have four equal spokes and close sight vanes. By around 1550 both Portuguese and Spanish makers had incorporated a cone shaped or later half round weight on the lower spoke to improve performance in the wind. My newest instrument ("mature") is of this design.

© R. Paselk 2013, Last modified 27 December 2020