How do sundials keep time? Why do sundials keep time?

Sundial. The original meaning is the shadow of the sun. The modern "sundial" refers to a kind of timekeeping instrument that used the shadow of the sun to measure the time in ancient times, also known as the "daily rule".

The principle is to use the direction of the sun's projection to determine and divide the time, which usually consists of a dial hand (watch) and a dial (a dial base). The method of using sundials to keep time is a major invention in the field of astronomical timekeeping, which has been used by mankind for thousands of years. Sundials can not only show the time of day, but also the solar terms and months.

Of course, its disadvantages are also obvious, it is bulky and cannot be used when there is no sunlight, such as cloudy days.

and at night. This invention has been used by human beings for thousands of years, until 1270 in Italy and Germany did not appear early mechanical clocks, and China in 1601 in the Ming Dynasty Wanli Emperor only got two foreign self-striking clocks, although there are many imported and self-made clocks in the Qing Dynasty, but they are used by the royal palace and noble mansions, and the common people still look at the dawn of the day.

How do sundials keep time?

During the course of the day, the shadows cast by the objects illuminated by the sun are constantly changing: the first is that the length of the shadow is changing. The shadow is longest in the morning, and as time goes on, the shadow becomes shorter, and after noon it becomes longer again;

The second is that the direction of the shadow is changing. In the Tropic of Cancer.

To the north, the morning shadow is in the west, the noon shadow is in the north, and the evening shadow is in the east. In principle, it is possible to time according to the length or direction of the shadow, but it is more convenient to time according to the direction of the shadow. Therefore, it is usually timed in the direction of the shadow.

Over time, the shadow on the sundial slowly moves from west to east. The shadow of the moving dial hand resembles the hands of a modern clock, and the dial face is the surface of the clock, which shows the time.

In the morning, the shadow fell on the west end of the plate.

Neighborhood; When the Sun reaches its highest position due south (Upper Midheaven), the shadow of the needle is located due north (lower), indicating the time of noon in the local area. In the afternoon, the sun moves westward, and the shadow of the sun slants eastward, pointing to the hours of Wei, Shen, and You.

The principle of sundial timekeeping: throughout the day, the shadows cast by the objects illuminated by the sun are constantly changing, first of all, the length of the shadow is changing, the shadow is the longest in the morning, and as time goes by, the shadow gradually becomes shorter, and it becomes longer again after noon; Then there is the change in the direction of the shadows, in the northern hemisphere, the morning shadows are in the west, the noon shadows are in the north, and the evening shadows are in the east.

In principle, it is possible to time according to the length or direction of the shadow, but it is more convenient to time according to the direction of the shadow. Therefore, Lu Lu is usually timed in the direction of the shadow. Since sundials must rely on sunlight, they cannot be used on cloudy days and nights.

Therefore, it is not enough to use a sundial to keep time, but also to match it with other types of timers, such as water clocks.

There are many types of sundials, each with different usages, and the specific usage is as follows:

Suppose the geographical latitude is , the height of the pointing hour hand is h, and the difference between the time to be carved and noon is t; If the angle between the timeline and the hour hand is a and the distance is d, then the calculation formula for each sundial is:

1. Horizon sundial: tana=tant*sin 2. Equatorial sundial: an equal disc, equivalent to 15 degrees per hour, and the noon line is facing vertically downward.

3. Polar sundial: d=h*tan(15t).

4. South-facing vertical sundial: tana=tant*cos 5. East or west vertical type: d=h*tan[15(6-t)]6

1) is the oblique angle of the sundial wall, tana=sino*tan(r+15t)2) refers to the angle between the hour hand and the wall perpendicular line w, tanw=sin *cot 3) refers to the angle of the hour hand high from the wall o, sino=sin *cos 4) refers to the angle between the hour hand and the timeline of the noon line r, cotr=cot *sin 5) the angle between the timeline at 6 o'clock and 12 o'clock s, cots=sin *tan 7, projection sundial:

1) The value of the time point in the direction of the major axis: d=sin(15t)2) The value of the time point in the direction of the minor axis: v=sin *cot 3) The ratio of the major axis to the minor axis of the ellipse: sin

4) The position of the vertical pole (human footing): d=tandel*cos, where del is the declination of the sun.

Comparing the timekeeping of sundials with the timing of clocks, after a period of observation, it will be found that:

Clocks and timekeepers have mechanical errors, while sundials have time differences.

Sundials can't tell the time at night and on cloudy days, but clocks can.

The time displayed on the sundial is the local time of the location, and the clock shows the standard time in the time zone.

The error of clocks and watches comes from the interior of imprecise machinery, and the error of sundials comes from the tilt between the earth's axis and the orbital plane.

There is a difference between the true solar time displayed on a sundial and the time displayed on a clock, which is known as mean time difference, and the maximum difference can reach a maximum of 15 minutes during the year. The cause of the mean time difference is that the Earth's orbit is elliptical, as well as the tilt between the axis of rotation and the orbital plane. Therefore, a good sundial must be accompanied by a permanent comparison table, at least in months, to correct the mean time difference.

Sundial timing is not entirely inaccurate, but it may not be precise in some cases. This is mainly due to several reasons:

1.The Earth's rotation is unstable: The speed of the Earth's rotation is not constant, but is affected by various factors, such as tidal friction, atmospheric resistance, etc. This causes periodic changes in the speed of the Earth's rotation, which affects the accuracy of the sundial.

2.Sun-Earth Distance Change: The Earth's orbit around the Sun is elliptical, so the Earth's distance from the Sun changes over the course of a year. This causes the angle of incidence of sunlight on the Earth's surface to change, which affects the timing of the sundial.

3.Atmospheric refraction: The Earth's atmosphere refracts sunlight, especially at low latitudes. Atmospheric refraction can cause errors in the position of the sun in the sky, which can affect the accuracy of the sundial's timekeeping.

4.Tilt of the Earth's axis of rotation: The Earth's axis of rotation has an approximate angle of inclination relative to the plane of the orbit of the revolution, which allows the Sun to move between the Tropic of Cancer in a year. This seasonal variation can affect the accuracy of the sundial.

5.Local Time and Standard Time: Sundials are timed according to local solar time, which is mostly used in modern life. The difference between the two affects the accuracy of the sundial's timekeeping.

In conclusion, although sundials can be used as timekeeping tools to a certain extent, their accuracy is limited due to a variety of factors, so they are not suitable for the needs of modern precision timekeeping.