The cross-staff is an instrument used to measure angles and altitudes, consisting of a trigonometrically graduated staff and one or more perpendicular vanes moving over it.

First described in about 1342 by the Jewish philosopher and scientist Levi ben Gerson and originally used for astronomical and surveying purposes, it became a mariner's navigational instrument in the 16th century (also known as 'ballastella', 'Jacob's staff ' or 'fore-staff').

The instrument was mainly used for finding the latitude by measuring the altitude of the polar star and for measuring the altitude of the sun. In the latter use, the observer had to face the sun and this inconvenience led to the development of the back-staff.

To measure the altitude of a celestial body, the eye-end of the staff was placed near the observer's eye and the other end half way between the horizon and the celestial body. The vane was then slid along the staff until its upper edge appeared to touch the celestial body, while the lower edge appeared to touch the horizon. The altitude could then be read off the staff.

W. F. J. M?rzer Bruyns, The Cross-Staff: History and Development of a Navigational Instrument (Zutphen, 1994).

Silke Ackermann
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