The origin and working principle of sundial (fifth grade science)

Humans have used sundials for a long time. The ancient Babylonians began to use it 6,000 years ago, and China in the Zhou Dynasty 3,000 years ago.

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.

The earliest reliable record in China is the short shadow level (horizontal sundial) invented by Yuan Chong in 594 AD (14th year of Sui Dynasty) mentioned in Tian Wenzhi. The clear record of the equatorial sundial was first seen in the shadow picture described in Volume 2 of Zeng Minxing's "Lonely Wake Magazine" in the Southern Song Dynasty.

2. Working 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. 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.

Extended data:

Classification of sundials:

According to the position, angle and area of the sundial, the sundial can be divided into horizontal, equatorial, meridian, unitary and vertical types, and its application scope is also different. According to the laying angle of masonry surface, it can be divided into horizontal type, vertical type and equatorial type.

1, horizontal sundial

Also known as the horizontal sundial, the surface of the horizontal sundial must be strictly horizontal? [4]? The included angle between brick surface and brick needle is the local geographical latitude. The calibration of masonry surface can only be determined by trigonometric function calculation. Suitable for use in low latitudes.

Because the sundial began by observing the direction change of the shadow of a vertical object in the sun, the earliest sundial may be a horizontal sundial. However, it must be described according to the actual situation. In the case of accurate timing with other timing tools, the surface shadow changes with time.

This is obviously inconvenient, but if the unified marking method is adopted, it will bring considerable timing error. This is a major defect of the horizontal sundial.

The horizon sundial is easy to manufacture and install. Besides being a general timekeeping tool, it can often be found in city squares, gardens, roads, communities, temples, schools, docks and tourist attractions, which is very unique and beautiful.

2. 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.

The clear record of the equatorial sundial was first seen in the shadow picture described in Volume II of Zeng Minxing's "Lonely Wake Magazine" in the Southern Song Dynasty, but the dial was made of wood. Later, it was changed to stone grate plate and metal grate needle. The Forbidden City in Beijing and other places have preserved the stone equatorial sundial made in Qing Dynasty.

3. Polar Mountains

The polar dial is parallel to the dial pointer, that is, the angle between the dial and the ground plane is the same as the geographical latitude and faces due north. The depiction of time can be handled by a simple geometric diagram, and the time axis of projection is a parallel line. Suitable for all latitudes. ?

4. Vertical sundial

Also known as Rich. It is a vertical sundial, and the dial is perpendicular to the ground plane. Britain, France, Germany, Italy, Austria and other European countries.

On the north-south (east-west) and east-west (north-south) walls of some gorgeous high-rise buildings in ancient and modern times, sundial sculptures with different colors, sizes and styles are attached.

Vertical sundials have four sides: south, north, east and west, which can be divided into three types: vertical sundials in the south, vertical sundials in the north and vertical sundials in the east and west.

A sundial with a dial facing due south and perpendicular to the ground. This kind of sundial is suitable for mid-latitude (30 ~ 70).

Calibrate the sundial vertically facing east or west (vertically facing east or west), with its disk facing due east or west and perpendicular to the ground. This sundial can only be used in the first half (east) or the second half (west), but it is suitable for all latitudes in the world.

5, projection sundial

The projection sundial does not have an hour hand, but only draws ellipses with different oblateness on the ground plane according to different geographical latitudes, and draws a time line on it, with the long axis pointing in the positive east-west direction and the short axis pointing in the north-south direction and engraved with the date, indicating the correct position of the vertical rod to measure the time. ? [6]?

6.folding sundial

Portable sundial is called folding sundial, which is made up of two facets connected by hinges. It is usually folded up and put in a small flat box, which is convenient to put in your pocket.

Dial hands are belts between two dials. When the belt is pulled and straightened, the two watch surfaces will be perpendicular to each other, with one surface forming a vertical sundial and the other surface being a horizontal sundial.

The folding sundial can be adjusted by combining two surfaces with different angles (which will produce different projections). When both sides of the sundial show the same time and the hinge is horizontal, the folding sundial indicates the local sun time.

In addition, when the hinge faces north (in the northern hemisphere), the folding sundial rotates at an angle so that the pointer is parallel to the earth's rotation axis, at noon, sunrise and sunset.

The time of the folding sundial will not be affected by the latitude change, but at 9: 00 am and 3: 00 pm, every time it deviates from 65438 0 degrees, the time difference between the two sides of the sundial will be 4 minutes (because the sundial is at the wrong latitude).

The folding sundial can be used like a compass and even measure latitude. Some folding sundials are attached with small plumb hammers and indexers to read the correct latitude; Some have compasses, which can measure geographical features. In ancient times, large folding sundials (1 m or more) were used for navigation.

7, such as high instrument sundial

The elevation sundial can show the correct date, which is used in navigation and astronomy. In design, it is a small flat ring with a small handle, just like an ornament on a watch chain, or a small ornament. ?

When the ring is hung by the handle, the small hole will cast a shadow on the inner side of the ring, and the time can be displayed by the mark in the ring, but the user must know whether it is morning or afternoon when measuring. Usually this small hole is set on a piece of metal and can be slid and locked to adjust the date.

References:

Baidu encyclopedia-sundial