For navigators on the deck of a moving ship, these limitations could be difficult to overcome. 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.
By having the sunward sighting vane cast its shadow on the lower sighting vane, it was possible to align the instrument to the sun. Care would have to be taken to ensure that the altitude of the centre of the sun was determined. This could be done by averaging the elevations of the upper and lower umbra in the shadow. In order to perform measurements of the altitude of the sun, a back observation quadrant was developed.
With such a quadrant, the observer viewed the horizon from a sight vane C in the figure on the right through a slit in the horizon vane B. This ensured the instrument was level.
The observer moved the shadow vane A to a position on the graduated scale so as to cause its shadow to appear coincident with the level of the horizon on the horizon vane. This angle was the elevation of the sun. Large frame quadrants were used for astronomical measurements, notably determining the altitude of celestial objects.
They could be permanent installations, such as mural quadrants. Smaller quadrants could be moved. Like the similar astronomical sextants, they could be used in a vertical plane or made adjustable for any plane. The sine quadrant was used for astronomical observations and solving trigonometry problems. The universal quadrant was used for solving latitude astronomical problems. The horary quadrant used the sun to find the time. The geometric quadrant is usually made of brass or wood, in a quarter-circle panel.
The earliest marine navigation geometric quadrants date to These quadrants had destination latitudes as opposed to degrees. The back observation quadrant was developed to allow for sun altitude measurements.
Quadrants were the most important and most popular tool used for navigation in the mids. It allowed the sailor to determine his position at sea by measuring the sun's angle over the horizon at noon. This made it possible to determine his latitude. There are several famous quadrants located in various places including Tycho Brahe's mural quadrant in Uraniborg, in Hven, the Greenwich Observatory mural quadrant, and the John Bird mural quadrant at the Mannheimer Observatory.
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