17th Century Back Staff

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£290.00

This is a wooden replica of a late 17th / early 18th century Back Staff.

The backstaff is a navigational instrument that was used to measure the altitude of a celestial body, in particular the sun or moon. When observing the sun, users kept the sun to their back (hence the name) and observed the shadow cast by the upper vane on a horizon vane. It was invented by the English navigator John Davis who described it in his book Seaman's Secrets in 1594.

The quadrant arc has been split into two parts. The smaller radius arc, with a span of 60°, is mounted above the staff. The longer radius arc, with a span of 30° is mounted below. Both arcs have a common centre. At the common centre, a slotted horizon vane is mounted. A moveable shadow vane is placed on the upper arc so that its shadow is cast on the horizon vane. A moveable sight vane is mounted on the lower arc.

In order to use the instrument, the navigator would place the shadow vane at a location anticipating the altitude of the sun. Holding the instrument in front of him, with the sun at his back, he holds the instrument so that the shadow cast by the shadow vane falls on the horizon vane at the side of the slit. He then moves the sight vane so that he observes the horizon in a line from the sight vane through the horizon vane's slit while simultaneously maintaining the position of the shadow. This permits him to measure the angle between the horizon and the sun as the sum of the angle read from the two arcs.

Since the shadow's edge represents the limb of the sun, he must correct the value for the semidiameter of the sun.

Our Quadrant is made from walnut treated with linseed oil

*Please not that these are made to order and can take up to a month to ship*




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