According to China's regulations, 1 nautical mile is equal to 1.852 km. However, this provision does not apply to all parts of the world. The length of the sea is not fixed, and different countries use different lengths. This is because, in navigation, the L-minute arc length of the earth's meridian circle is defined as 1 nautical mile (that is, the meridian length corresponding to 1 degree latitude). But because the earth is not a standard sphere, the L-arc length of meridian circle is different at different latitudes.
When sailing at sea, it is difficult to measure distance, speed and position because there is no reference object to provide reference. Before latitude and longitude were applied to navigation, navigation was generally based on star map and compass; By about 1500, nautical instruments were invented and latitude and longitude began to be added to charts.
/kloc-in the 6th century, maritime navigation was quite developed, but at that time there were no clocks or voyage recorders, so it was difficult to accurately determine the sailing speed of the ship. However, a clever sailor came up with a wonderful method. When the ship is sailing, he throws a floating body with a rope into the sea, and then calculates the speed of the ship according to the length of the rope pulled out in a certain period of time. The time at that time was still measured by quicksand timer. In order to calculate the speed of the ship more accurately, sometimes the rope is very long, so many knots are tied at the equal distance of the rope, so that the whole speed measuring rope is divided into several sections. As long as the number of knots dragged by the rope in the same unit time is measured, the corresponding speed can naturally be measured. 16 17, Dutch scientist Snell estimated the circumference of the earth with 24,630 Roman miles (24,024 miles). After that, the British mathematician Edmund? Gunter improved its navigation tools, suggesting that latitude and longitude should be the basic unit for measuring distance. ?
Divide the circumference of the earth by 360 to get the distance per degree, and then divide it by 60 to get the distance per minute, which is the sea. Because the shape of the earth is not a perfect sphere, and the dimension distance of 1 is not constant, countries have different standards for calculating the sea, but there is little difference. Although the current navigation equipment is very advanced and developed, it has been preserved because the sea has become a idiom.