linguistic:paraphrasing
Great circles on Earth

Two units of length, the nautical mile and the metre (or kilometre), were originally defined so the lengths of arcs of great circles on the Earth's surface would be simply numerically related to the angles they subtend at its centre. The simple equation s = θ {\displaystyle s=\theta } applies in the following circumstances:

The lengths of the distance units were chosen to make the circumference of the Earth equal 40 000 kilometres, or 21 600 nautical miles. Those are the numbers of the corresponding angle units in one complete turn.

Those definitions of the metre and the nautical mile have been superseded by more precise ones, but the original definitions are still accurate enough for conceptual purposes and some calculations. For example, they imply that one kilometre is exactly 0.54 nautical miles. Using official modern definitions, one nautical mile is exactly 1.852 kilometres, which implies that 1 kilometre is about 0.539 956 80 nautical miles. This modern ratio differs from the one calculated from the original definitions by less than one part in 10,000.