Backstaff Instruments

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The Cross Staff
The Cross Staff was used to measure the angle between the Sun or the North Star and the Horizon.
This instrument was used from the 15th through 17th centuries.

In principle, the cross staff is a very simple device to understand and operate. The short cross piece slides along the staff. One end of the staff is placed at your eye. The cross piece is slid forward or back along the staff until the lower edge of the cross piece touches the horizon and the upper edge just touches the North Star or the lower limb of the sun. Then the angle, in degrees between the object and the horizon is read off the scale which is etched onto one long side of the staff. That angle is easily translated into your latitude. That's the principle.

Various errors crept into the operation not the least of which is the extreme difficulty of looking at two places at once - the lower and upper edge of the crosspiece - parallax error. At the same time you were trying to slide the cross piece to just the right location. Add a pitching rolling deck to the equation and you can see there's significant error. Also, a user won't place the staff at the same point near the eye in consecutive readings. Lastly, any play in the mortice in the cross piece meant error.

Quite often, the other side of the staff would have been marked off in inches, eights and sixteenths. This would allow the user to calculate the distance away from an object of known height using simple trigonometry.

Cross staffs were equipped with several cross pieces of different lengths; this allowed sightings at a wide range of latitudes.

Cross staffs weren't very accurate, but they were better than no instrument at all.