Introduction: Units of length, commonly used in our daily lives include "meters", "centimeters", "kilometers", etc. We are all familiar with their unit lengths.

And there are also some things we often mention, such as "the sea", but it seems unclear how long it actually represents?

How long is a nautical mile? At this point, we will also have another question: why should the measurement unit at sea use the word "nautical mile", and why cannot it be unified with "kilometer" just like on land?

　　

 

1、 The Sea

1. Concept Origin

We usually use a measuring ruler to measure the length of an object. If there is no measuring ruler, we will instinctively observe the objects around it and compare their sizes. By comparison, we can know its approximate length and other numerical values.

On land, it is relatively easy for us to estimate the length of any physical object in this way because there are many reference objects. And how can we do it on the sea surface?

　　

 

Where is the reference point at sea? We may associate ships, islands, reefs, birds, etc. However, compared to the vast sea, it can be said that it is still difficult to encounter and use it as a reference for comparison.

Moreover, at sea, we cannot remain stationary like on land. The speed of movement at sea is not artificially controlled. At this point, the speed of the sea cannot be determined.

Various complex factors are intertwined, which has forced predecessors to explore and solve these problems. The "sea" was also born during this long process of research and exploration.

　　

 

In the fifth century BC, our predecessors already had the ability to sail far away. In their continuous practice, they guessed that the seemingly flat sea surface actually presents an arc shape.

So, can we speculate that the Earth where humans are located is actually a "circle" or "sphere"?

　　

 

In the 16th century, with the continuous development of navigation technology, more and more people explored it. After the continuous advancement of theory and practice, the latitude and longitude of the sea were measured using navigational instruments, and the concept of "meridian circles" emerged.

The so-called "meridian circle" is the big circle drawn by any point on the earth passing through the north and south poles. Each circle is a circle, and each circle is 360 degrees. It needs to be divided into 360 parts, each part being 1 degree, and then divided into 60 parts, each part being 1 point. This is the distance of one nautical mile.

　　

 

2. Unit length

So, the concept of "sea" has emerged, and length is the second important issue that needs to be clarified next. How long is "One Sea"? At this point, it is necessary to connect with the measurement standards of the "metric system".

In the 18th century, France officially adopted the "metric system" to measure the length of objects. The length standard of "one meter" set by them at that time was one millionth of the length of the quarter circle from the North Pole to the equator, passing through the meridian of Paris.

However, at this point, the length of one meter is not precise enough, and there may still be some deviations when using it. So, later researchers further clarified it and combined it with the measurement of the speed of light.

　　

 

Therefore, the length of "one nautical mile" can be calculated. The length of the meridian circle is four times the length from the North Pole to the equator, which is 40 million meters or 40000 kilometers. By applying the calculation method mentioned earlier, the length of one nautical mile can be calculated.

Since the Earth is not an absolutely perfect sphere, its meridians are not of equal length. The meridian circle is not of equal length, and the length of a nautical mile is not equal for every value. One nautical mile, the longest is 1.862 kilometers, and the shortest is 1.843 kilometers.

A sea has its own length and length, and even if accurate data is obtained, it is not convenient for us to use it. In 1929, the International Conference on Hydrography unified the unit length of the sea, 1.852 km/sea mile.

　　

 

2、 The appearance of 'nautical mile' occurs earlier than 'kilometer'

The use of "nautical mile" as the unit of measurement in the ocean and "kilometer" on land is a well-established rule in our current practice, as it is.

Moreover, after understanding the origin of "sea", we also know that the concept of "sea" began a long time ago, earlier than the appearance of "kilometer", and its birth is related to the shape and volume of the Earth. So why use "kilometers" instead?

　　

 

Summary: The birth of a name usually has its source and is also derived in chronological order. In general, the name 'latecomer' is not used to replace 'previous person'.

However, we live on land and most of the time we come into contact with people and things on land. Therefore, compared to the "sea", we are more familiar with the "kilometer".

Now that we have understood and distinguished the two, we can understand why the unit of measurement at sea is "nautical mile" instead of "kilometer".