What do you think of the sundial?

The upper end of the needle points to the north celestial pole and the lower end points to the south celestial pole. 12 cubes are carved on both sides of the brick surface, and each cube represents two hours. When the sun shines on the sundial, the shadow of the sundial needle will be cast on the surface of the sundial, and the sun will move from east to west, and the shadow cast by the sundial needle on the surface will slowly move from west to east. Therefore, the moving shadow of the coffin needle seems to be the pointer of modern clocks and watches, and the coffin surface is the surface of clocks and watches to show the time.

Brief introduction of sundial:

The original meaning of the sundial refers to the shadow of the sun, which is a device that uses the position of the sun to measure time. It is mainly composed of a pointer that projects the sun's shadow, a projection surface (i.e., a dial surface) that carries the pointer's projection, and scale lines on the dial surface. The most common design is the garden sundial, and the shadow of the sun is projected on the plane marked with time. When the sun moves, the time indicated by the shadow changes. In fact, a sundial can be designed on the surface of any object, so that a fixed pointer can produce a shadow to measure time. So there are many different forms of sundials, which can be basically divided into horizontal sundials, equatorial sundials, meridian sundials and unitary sundials. Most sundials show the apparent solar time, and some sundials have been changed in design, which can show the average solar time or daylight saving time.

Second, the principle:

During the day, the shadows cast by objects illuminated by the sun are constantly changing: first, the length of the shadows is changing. The shadow is the longest in the morning. As time goes on, the shadow becomes shorter and longer after noon. The second is that the direction of the shadow is changing. In the northern hemisphere, the shadow of morning is in the west, the shadow of noon is in the north, and the shadow of evening is in the east. In principle, it can be timed according to the length or direction of the shadow, but it is more convenient to time according to the direction of the shadow. Therefore, the timing is usually based on the position of the shadow. ? [4] With the passage of time, the shadow on the needle moves slowly from west to east. The moving shadow of the hand seems to be the pointer of a modern clock, while the surface of the hand is the surface of the clock to show the time. In the morning, the shadow is cast near Shi Mao at the western end of the disc; When the sun reaches the highest position due south (upper meridian), the needle shadow is located due north (lower), indicating the local noon time. In the afternoon, the sun moves west and the shadow leans east, pointing to each hour in turn.