b.cross-staff and backstaff
Cross-staff
Cross staff, dated:1571 ;Wood and gilt brass; 1393 mm in length
British Museum, London.
The pictures show medieval surveyors using the cross-staff
The
cross-staff is a mariner’s navigational instrument in the 16th century. It is
also known as ballastella, Jacob’s 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 observer’s 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.
Back staff
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