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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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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.
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. There are three vanes - the horizon vane (A), the shadow vane (B) and the sighting vane (C). The sun casts that shadow of a vane (B in the adjacent image) on a horizon vane (A). The horizon vane has ...
The backstaff, which measures the length of a shadow, was used from the 16th century and saw iterative improvements such as the Davis quadrant. These were in use in parallel with the octant and early sextant; the sextant eventually displaced the others, and is still used to this day.
The triangular frame at the top spans 60° as did the arc on the backstaff. The main graduated arc subtends 30° as in the backstaff. The 30° arc is graduated in degrees and sixths of a degree, that is, at ten-minute intervals. The sighting vane of the backstaff is replaced with a sight (called an eye vane [2]) mounted on the end of the index arm.
From 1601 to 1603 he accompanied Sir James Lancaster as Pilot-Major on the first voyage of the English East India Company. For his part Davis was to receive £500 (around £1.5 million at 2015 values [ 7 ] ) if the voyage doubled its original investment, £1,000 if three times, £1,500 if four times and £2,000 if five times.