Backstaff Instruments

16 Stonybrook Road

Marblehead, MA 01945

Phone: (781) 639-1694

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The Quadrant
The quadrant was used to find latitude by measuring the angle between the horizon and the North Star or Sun. It was also used to find the distance away from an object of known height. Amazingly enough, the quadrant was also an excellent ANALOG COMPUTER!
This instrument was used in the 15th and 16th centuries.

The Quadrant was a major advance in instrument design over the cross staff because it eliminated the parallax error. The double pinhole sight made for more accurate and repeatable sights. However unlike the backstaff, one still had to look directly at the sun to make a sun sight. The quadrant got it's name because it could measure angles of up to 90 degrees: one quarter of a circle.

Operation of the quadrant was simple. The user sighted through the two pinholes along one edge of the quadrant, at either the North Star or the Sun. A string from the apex always pointed towards the center of the earth as there was a weight at the end of the string. As the user tilts the quadrant to sight the object, the angle was read off the point where the string intersected the degree scale alonge the curved bottom of the instrument.

Like the cross staff, the quadrant could be used to measure the distance away from an object of known height using the tangent/cotangent scales near the apex of the device and a little trig.

Perhaps the most interesting aspect of this instrument was the semi-circle scale drawn from the apex to the zero degree mark (with the center of the circle on the flat edge of the device). This semi-circle was used in conjunction with the weighted string, a bead which slid up and down the string, and a scale along the sighting edge to allow the user to calculate the number of nautical miles (or leagues if desired), per degree longitude at any latitude.

And no power plug was required for this computer!