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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.
In navigation the instrument is also called a cross-staff and was used to determine angles, for instance the angle between the horizon and Polaris or the sun to determine a vessel's latitude, or the angle between the top and bottom of an object to determine the distance to said object if its height is known, or the height of the object if its distance is known, or the horizontal angle between ...
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The son of Willem Fredrik Jacob Mörzer Bruyns, Sr. (1913–1996), a Dutch merchant mariner, and a naval officer (reserve), [1] the young Willem Mörzer Bruyns initially trained as navigation officer at the [Amsterdam Nautical College] (Hogere Zeevaartschool van het Zeemanshuis), Mörzer Bruyns sailed as a junior officer with the Amsterdam-based Netherland Line.
Navigators had been able to work out latitude accurately for centuries by measuring the angle of the Sun or a star above the horizon with an instrument such as a backstaff or quadrant. Longitude was more difficult to measure accurately because it requires precise knowledge of the time difference between points on the surface of the Earth.
Drawing of a back observation quadrant. This instrument was used in the manner of a backstaff to measure the elevation of the sun by observing the position of a shadow on the instrument. In order to avoid staring into the sun to measure its altitude, navigators could hold the instrument in front of them with the sun to their side.
Seventeenth century marine navigation and cartography used the "backstaff" which, in this area, was accurate to one degree of longitude, or around 60 nautical miles. The Edinburgh Gazetteer, or Geographical Dictionary published in 1827 also mentions the island and provides a geographical position. [ 3 ]
The backstaff is signed in the lower right corner: fecit J.Sisson (made by J. Sisson). [20] In 1732 Sisson was selected to make a brass octant to John Hadley's new design. The instrument proved reliable and easy to use in sea trials, even though weather conditions were poor, and was clearly an improvement over the cross-staff and backstaff. [2]