cross-staff

English - English

Definition of cross-staff in English English dictionary

An early navigational instrument, consisting of a wooden rod with a sliding crosspiece, used for measuring the altitude of a star; it developed into the sextant

An early sixteenth-century instrument for measuring the altitude of a heavenly body It consists of a square shaft and a sliding cross-piece set at right angles to the shaft The shaft end is held at the observer's eye and the cross-piece positioned to line up with the sun and the horizon The cross-piece marks a point on the shaft that is referred to in a table of degrees and minutes

a navigation instrument for finding latitude, it was eventually replaced by the backstaff which didn't require looking at the sun to use

A navigational aid, the most accurate device of its kind, probably invented in the east and now used widely by pilots throughout western seas The cross staff consists of a square rod, approximately three feet in length Each of the rod's four faces are engraved with a different scale of degrees Four short cross-pieces of different length are used, depending on the size of angle that is to be measured Each crosspiece is designed for one of the four scales " The observer points the rod roughly midway between the object of the sight and the horizon, and moves the cross-piece until it exactly fits between them He then reads the altitude from the appropriate scale

Related Terms

crossstaff

An instrument formerly used at sea for taking the altitudes of celestial bodies

crossstaff

A surveyor's instrument for measuring offsets