The principle of sundial timing

The sundial can display not only the time of day, but also the solar terms and months. Of course, its shortcomings are also obvious. It is huge and cannot be used when there is no sunshine, such as cloudy days and cloudy nights.

This invention has been used by human beings for thousands of years. It was not until 1270 that early mechanical clocks appeared in Italy and Germany, while in China, it was not until 160 1 year that Emperor Wanli of the Ming Dynasty got two foreign chimes. Although there were many imported and homemade clocks and watches in the Qing dynasty, they were all used by the palace, and ordinary people still watched the dawn.

The working principle of sundial

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. North of the Tropic of Cancer, the shadow in the morning is in the west, the shadow at noon is in the north, and the shadow at night 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. As time goes on, the shadow on the needle slowly moves 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.

Classification of sundials

Horizontal sundial

Also known as the horizontal sundial, the horizontal plane of the sundial must be strictly horizontal, and the angle between the surface and the sundial pointer is the local geographical latitude. The calibration of masonry surface can only be determined by trigonometric function calculation. Suitable for use in low latitudes.

Equatorial sundial

The surface of the equatorial sundial is the equatorial plane. According to the latitude of the place of use, the surface of the equatorial sundial is parallel to the equatorial plane. The scales on the dial are equally divided, and the shadows of the summer and winter axes projected on the dial will be divided into the north and south parts of the dial. Suitable for middle and low latitudes. If the dial is changed into a ring, it is called the equatorial compass sundial.

The equatorial sundial is also called the oblique sundial. The equatorial sundial is the most important and common sundial in the world and the most classic and traditional astronomical observation instrument in ancient China.