When the early navigators sailed along the route in the sea, they needed to constantly determine the location of the ship, that is, the intersection of the longitude and latitude of the ship. In order to figure out the latitude of the ship, the navigator needs an instrument that can measure the angle between the horizon and the midday sun, or by measuring the angle between the horizon and a fixed star. To determine the latitude. Initially, sailors used the astrolabe to measure the height of the sun, but since the deck of the ship was up and down, the instrument was extremely difficult to operate and was not easy to measure accurately. Later, people replaced the astrolabe with a right angle instrument. (figure) Sextant on the nautical chart and early navigation instruments such as two-foot gauge, protractor and parallel ruler
In 1731, Hadley invented the Reflective Quadrant and quickly developed the sextant, a bow-shaped instrument that measures 1/6 of the circumference. Hadley and Harley have developed a reflective telescope. He then created an instrument that measures the angle at sea. The observer can see both the horizon and the sun through a mirror, and the angle between them is measured by a quadrant with a scale on the edge. In 1732, the British Admiralty placed a 90-degree quadrant on a speedboat for testing, and the results were very accurate. Then the quadrant became an essential instrument for naval navigation. In 1757, Captain Campbell expanded the curvature of the quadrant to measure the angle of 120 degrees, so that the quadrant became a sextant. It consists of a triangular frame with a curved plate on one side, a scale on the top and a pointer that can be moved. The mirror reflects the two objects to be measured at the angle, so that the angle can be easily measured and the latitude of the ship can be calculated to ensure that the ship travels along the correct route.
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