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Backstaff is the name given to any instrument that measures the altitude of the sun by the projection of a shadow. It appears that the idea for measuring the sun's altitude using back observations originated with Thomas Harriot. [2] Many types of instruments evolved from the cross-staff that can be classified as backstaffs. Only the Davis ...
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 ...
An almucantar staff is an instrument chiefly used to determine the time of sunrise and sunset, in order to find the amplitude and consequently the variations of the compass. Usually made of pear tree or boxwood, with an arch of 15° to 30°, [3] it is an example of a backstaff. This is a drawing of an almucantar staff.
The frame of a sextant is in the shape of a sector which is approximately 1 ⁄ 6 of a circle (60°), [2] hence its name (sextāns, sextantis is the Latin word for "one sixth"). "). Both smaller and larger instruments are (or were) in use: the octant, quintant (or pentant) and the (doubly reflecting) quadrant [3] span sectors of approximately 1 ⁄ 8 of a circle (45°), 1 ⁄ 5 of a circle (72 ...
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.
This instrument clearly reflects the shape and features of the Davis quadrant. The significant differences are the change in the upper arc to a simple triangular frame and the addition of an index arm. The triangular frame at the top spans 60° as did the arc on the backstaff. The main graduated arc subtends 30
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]
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.