Why can lotus lettering be made simply and accurately?

The arrow carved by Yan Su, a scientist in the Northern Song Dynasty, is made of wood. Because of the buoyancy of water, it can pass through the lotus plumule and rise along the diameter. There is a scale on the arrow, from which you can see what time and solar terms are. According to the subtle differences in the length of day and night in a year, the 24 solar terms are made into 48 floating arrows with different scales, and each solar term is replaced day and night. This kind of notch is simple to make, accurate in timing, exquisite in design and easy to popularize.