Davis's quadrant

Davis's quadrant, or back-staff, named by the French the English quadrant. John Davis (c.1550–1605), the illustrious English navigator and explorer of Elizabethan times who mounted three voyages in search of the North-West Passage, invented this simple instrument for measuring the altitude of the sun which obviated the need of sighting the sun direct. It consists of a graduated staff on which is fitted a half transom in the form of an arc of a circle which could be slid along the staff. At the fore end of the staff is a horizon vane with a slit through which the horizon could be observed. In use the staff was held horizontally, the observer having his back to the sun. The half transom was then moved along the staff to a position at which the edge of its shadow struck the horizon vane and coincided with the horizon viewed through the slit in the vane. The staff was graduated to 45° and the instrument was useful for measuring altitudes of the sun of less than about 45°.

For measuring greater altitudes Davis invented a 90° back-staff which used two half transoms, one straight, the other an arc. The straight half transom was fitted perpendicularly to, and stood vertically above, the staff when the instrument was in use. It was designed to slide along the staff. The arcuate half transom was fixed to the lower side of the staff and provided with a sighting vane. The fore end of the staff was fitted with a horizon vane through which the horizon could be sighted.

To take a sight with the 90° back-staff the straight half transom was set at a graduation on the staff corresponding to a few degrees less than the sun's altitude. The instrument was then held in the vertical plane and the observer, with his back to the sun and his eye at the sighting vane, slid the latter to a position on the arc so that he was able to see the horizon through the slit in the horizon vane coincident with the shadow of a shadow vane fitted at the top of the straight half transom. The sun's altitude was the sum of the angles indicated on the staff and arc respectively.

Before the end of the 17th century the back-staff had all but replaced the seaman's quadrant, the astrolabe, and the cross-staff; and it was not superseded for sea use until about 1731 when Hadley's reflecting quadrant was introduced. The quadrant was not capable of adjustment and it was therefore necessary for the user to ascertain in advance its instrumental error. This was usually done by making meridian altitude observations at places of known latitude. Having thus discovered the error, the observer applied it to all altitudes measured with it. According to whether the error tended to increase or decrease the ship's northerly latitude the quadrant was said to be northerly or southerly.

Mike Richey