b.cross-staff and backstaff
Cross staff, dated:1571 ;Wood and gilt brass; 1393 mm in length
British Museum, London.
The pictures show medieval surveyors using the cross-staff
cross-staff is a marinerís navigational instrument in the 16th century. It is
also known as ballastella, Jacobís staff or fore-staff.
Instead of measuring the celestial body (usually, the pole star or the sun) down from zenith, it measures the angles and altitude of the celestial body from the horizon (as this is usually a clear cut line on fine days). It has a graduated staff with one or more perpendicular vanes moving over it.
To measure the altitude of a particular celestial body, the eye-end of the staff will be placed near the observerís eye and the other end will the half way between the horizon & the celestial body. The vane is then slid along the staff until its upper edge appear to touch the celestial body while the lower edge touches the horizon.
The altitude can be easily read of the staff.
In measuring the altitude of the sun, the observer had to face the sun, which may be quite inconvenient and not to mention bad for the eyes. Hence this led to the development of the back staff.
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