What would the ancients do if they wanted to know the size of the horn?

It is recorded in the history books that Dayu used accuracy, rope, rules and moments to control water. Therefore, when China had the concept of words or numbers, he already had the concept of angle: using a level to determine the absolute plane, and using a plumb rope and a ruler to measure the parameters of the angle (somewhat similar to using trigonometric functions to represent angles, not modern angles).

"Plane angle is the inclination of two intersecting straight lines in a plane but not on a straight line" appeared in Euclid's Elements of Geometry, in which Euclid made a perfect definition of diagonal, and even defined right angles, acute angles and obtuse angles. It can be seen that the west also has corresponding measuring tools to measure angles. Moreover, the measurement result in the west is probably directly the size of the angle-the initial radian angle is aimed at astronomical astrology, which is very different from our country.