Replicas and Restorations of Early Instruments by Richard A. Paselk

Richard Paselk, Curator

The Navigator's Cross-Staff

The cross-staff (also known as the baculus or Jacob's Staff) was one of the earliest navigation instruments devised by Henry the Navigator's school of navigation. As seen in the illustration above it consists of a graduated staff and on or more sliding vanes. In use one end of the staff is held next to the eye while the vane is adjusted so that the ends of the vane aligns with the horizon and a celestial object (to determine altitude). Alternatively the two ends are aligned with different celestial objects to determine the angular distance between them.Note that the ends are generally distinguished so the proper end is held to the eye in the dark (mine has a copper cap at the eye end, others may be pointed at the opposite end).

The origins of the instrument are controversial. In China cross-staffs are described for use in surveying as early as 1086, while in the West the first description is by the astronomer Levi ben Gerson in 1342 for use in astronomy. Alternatively it may have been derived from observing Arab pilots using the Kamal* during the voyages of Vasco de Gama to the Indian ocean.In any case the Portuguese introduced it for maritime use around 1515, with the first written reference to this use just before 1520. The initial instrument had a single scale on one side graduated from 0–90° for determining the altitude of the pole star. The use of the instrument migrated to Northern Europe by the mid 16th century where it was further developed, particular by the English and especially the Dutch. All four sides of the staff became graduated with different ranges, and sometime a zenith scale. Additional aides, such as filters to allow viewing the Sun were also added. Those interested in further details are referred to Mörzer-Bruyns exceptional work on the cross-staff in the References section.**

* The kamal consists of a wood card on a string graduated with knots at specific distances giving the altitudes of known stars as seen from specific ports at specific times of the year.

**Mörzer-Bruyns, W.F.J. The Cross-Staff: History and Development of a Navigational Instrument. Vereeniging Nederlandsch Historisch Scheepvaart Museum, Amsterdam (1994). This book gives a very thorough description and history of the cross-staff, a modest but very important navigational instrument. Just as important for the model builder the author describes the layout of typical instruments, and describes all of the known extant examples, with dimensions, markings etc. The definitive work on these instruments.

© R. Paselk 2013, Last modified 26 December 2020