In the early 16th century, long-distance navigation swept through Western societies as various governments dispatched their fleets searching for the legendary lands abundant in silk, spices, tea, and porcelain.

This marked the beginning of the global Age of Discovery.

However, sailors quickly encountered a fatal issue during ocean voyages – the speed of sailing ships was challenging to calculate in the vast sea without reference points. The inability to measure a ship's speed quickly led to deviations in the intended course, resulting in many ships being lost at sea without the possibility of returning.

Since the sun and stars are constantly in motion, and with no fixed landmarks at sea, navigators had to use the Earth as a reference frame. In the mid-16th century, Magellan's expedition circumnavigated the globe, confirming the long-held hypothesis that the Earth is a sphere. With this knowledge, people developed the concept of latitude and longitude.

Within the same circle, equal angles correspond to equal arc lengths. Applied to the ocean, this principle means that if a ship sails along the same latitude, the distance traveled northward remains constant. The Earth's meridian was divided into 360 equal parts, each representing one degree.

Each degree was further divided into 60 parts, and one of these parts corresponded to one nautical mile. This allowed seafarers to determine their position by calculating their latitude and longitude, enabling them to calculate a ship's speed.

Understanding the origin of nautical miles, we can now calculate their length. Considering the Earth as a perfect sphere, the circumference of a meridian is the same as the equator's, measuring over 40,000 kilometers. Dividing this meridian into 21,600 parts (360x60), we get a value of approximately 1,851 meters for one nautical mile.

However, a new problem arises because the Earth is not a flat circular shape but a three-dimensional sphere. The distances between meridians vary, more significant at the equator and minor near the poles. Consequently, the value of a nautical mile is not entirely fixed, with a difference of about twenty meters between the maximum and minimum values.

Countries also have different nautical mile values due to variations in geographical locations and latitudes, ranging from the shortest at 1,843 meters to the longest at 1,862 meters.

Having learned about the origin of the "nautical mile," we also understand that this concept predates the "kilometer" and is intricately linked to the shape and volume of the Earth. So, why did the term "kilometer" replace it?

The creation of names typically has its origins and follows the chronological order of time. Generally, a later term is not used to replace an earlier one. However, human activities primarily occur on land, and people predominantly interact with things on land. Therefore, compared to the "nautical mile," people are more familiar with the "kilometer."