Why are distances at sea expressed in nautical miles instead of kilometers?

We are already very familiar with the commonly used units of length such as "meters", "centimeters" and "kilometers". However, most people are not very clear about the distance represented by the "nautical mile", a unit of measurement for sea navigation. At this point, another question arises: Why do we use nautical miles to describe distance at sea instead of kilometers?

The Origin of the Concept of “Nautical Miles”

When measuring the length of an object, a ruler is usually used. If there is no ruler, people will instinctively observe the objects around it and compare their sizes. By comparison, they can know its approximate length and other values. On land, it is actually easy for us to estimate the length of any physical object in this way, because there are many reference objects. But how can we do it on the sea? You may think of ships, islands, reefs, flying birds, etc. Compared with the vast ocean, it is more difficult to use them as reference objects. Moreover, on the sea, it is impossible to stay still like on land. And the speed of movement on the sea cannot be controlled artificially. At this time, the speed of moving on the sea cannot be determined. Various complex factors are intertwined with each other, which prompted our predecessors to explore and solve these problems. The concept of "nautical mile" was born in this long process of research and exploration.

How long is “one nautical mile”?

In the fifth century BC, our ancestors had the ability to sail in the ocean. In the process of their continuous practice, they guessed that the seemingly flat sea surface was actually in the shape of an arc. In the sixteenth century, with the continuous development of navigation technology, more and more people explored this. After the continuous advancement of theory and practice, the concept of "meridian circle" appeared.

The so-called "meridian circle" is a great circle drawn by any point on the earth, passing through the North and South Poles . Each circle is a circle, and one circle is 360°. It needs to be divided into 360 parts, each part is 1°, and then divided into 60 parts, each part is 1 point. This is the distance of one nautical mile.

Then, the concept of "nautical mile" came into being, and length is the second important issue to be clarified. At this time, we have to contact the "meter system" of weights and measures. 1 kilometer is 1 kilometer, which is familiar to us, and the definition of meter can be traced back to 1799, when France officially began to use a measurement standard called "meter system", which defines the length of 1 meter as: "1/10 million of the distance from the North Pole of the Earth to the Equator of the Earth through the Paris meridian". In the following time, the "meter system" has gradually been widely adopted. In order to accurately express the length of 1 meter, people have tried every possible way, but there will always be a certain error in practical applications. This problem was not solved until scientists were able to measure the speed of light very accurately. So the length of 1 meter was finally defined as: "The distance that light travels in a straight line in a vacuum in 299792458 seconds".

Now let's assume that the earth is a perfect sphere. According to this standard, the distance from the North Pole to the equator on the meridian of Paris is actually one-fourth of the circumference of the meridian circle mentioned above, so the circumference of the meridian circle is 40 million meters, or 40,000 kilometers. At this time, the length of one meter was not accurate enough, and there would be some deviation when using it. Therefore, researchers later made it further clarified and combined it with the speed of light for measurement. Therefore, the length of "one nautical mile" can be obtained through calculation. The length of the meridian circle is four times the length from the North Pole to the equator, which is 40 million meters, or 40,000 kilometers. Then, applying the calculation method mentioned above, the length of one nautical mile can be calculated. According to the definition of a nautical mile, it can be calculated that 1 nautical mile is equal to 40 million meters divided by 360 and then divided by 60, and the result is about 1,852 meters, that is, 1 nautical mile is about 1.852 kilometers.

However, the earth is not a perfect sphere, which means that there will be certain differences in the distance of 1 nautical mile in areas of different latitudes on the earth . For example, in the equatorial region, 1 nautical mile is approximately 1.843 kilometers, while in the polar regions, 1 nautical mile is approximately 1.862 kilometers.

In this case, different regions on the earth have different definitions of "how many kilometers is 1 nautical mile", but generally speaking, people generally adopt the standard adopted by the International Hydrographic Conference in 1929. According to this standard, 1 nautical mile is 1.852 kilometers.

Source: China Ocean Development Research Center

"Ocean Archives" Popular Science