Why does our clock only say 12 hours instead of 24 hours?

Record children's 1, whys and protect children's inquisitive nature.

A child in the rainforest class asked a question:

Why is our clock only marked with 12 hours instead of 24 hours?

Now, almost all countries have adopted the 24-hour system.

Since there are 24 hours a day

Before we talk about the clock problem, let's talk about a geographical knowledge: from midnight to 12 noon, the solar altitude angle is from degrees to 9 degrees, and then decreases in turn, and finally returns to midnight, that is, it returns to degrees.

It turns out that in ancient times, people used the movement of the sun to measure time. But there is no sun at night, so we have to measure 12 hours during the day. In ancient times, according to the sun, the shadow of the things it illuminates will also move with it. By observing this natural phenomenon, I learned to calculate the time and discovered the sundial. In this way, we continued the habits of the ancients, and there will be 12 o'clock now.

Besides, if the clock is 24 hours, the scale of every hour is only 15 degrees, which is not convenient for our observation, and the mechanical difference will be greater, the visual difference will be greater, and if it is a watch, it will look denser. In 12 hours, the scale of every hour is 3 degrees, and the sense of consciousness will be better.

In p>79, Canadian railway engineer Fleming put forward the concept of "time zone", which was recognized at an international conference in 1884, thus formally establishing a "time zone system" to unify the world measurement time. The "time zone system" stipulates that every 15 longitude range on the earth is regarded as a time zone (that is, the longitude that the sun travels within one hour). The whole earth's surface is divided into 24 time zones. The "central meridian" of each time zone is defined as (that is, the "prime meridian"), 15 east-west longitude, 3 east-west longitude, 45 east-west longitude ... until 18 longitude, at all locations within the range of 7.5 east and west sides of each central meridian, The local time of the central meridian is always used as the standard time.

The division of this world time zone is based on the prime meridian. It is a zero area from 7.5 west to 7.5 east longitude (with a longitude interval of 15). From the two boundaries of the zero-time zone, one time zone is drawn east and west respectively, and 12 time zones are drawn east and west, and the 12-hour zone in the east and west coincides with the 12-hour zone in the west. * * * is divided into 24 time zones in the world. Each time zone takes the local time of the central meridian as the local standard time.

This is artificially stipulated. If people stipulated 1 at that time, it might be 1 now.

Because 24 hours are divided into day and night, the numbers on the clock are divided into ~12.

Many of them are related to astronomical phenomena, such as one year (the earth revolves around the sun). However, there are also many things that have nothing to do with astronomy, which are formulated purely because of certain humanistic factors, such as folk customs, politics, religion, etc., for example, seven days a week, 24 hours a day, etc., all of which have nothing to do with astronomical phenomena. We generally regard the system of dividing a day into 24 hours as an Egyptian invention. However, if you want to ask, why 24 hours? Instead of 18 hours? Or 26 hours? Hehe, this may only go back to ancient Egypt and ask the ancient Egyptians! Since the Song Dynasty (about 1 years ago), every hour in China has been subdivided into two parts: the beginning and the ending. For example, the beginning refers to an hour between 23: and : midnight, and the ending refers to an hour between : midnight and 1: am. This portable equatorial sundial divides the time from noon to noon the next day into 24 periods according to "early" and "positive", which coincides with the current system, hence the name "hour". About 5 years ago (AD 154), Walt, a westerner, invented the mechanical clock. In 1656, after c. huygens added the pendulum to the mechanical clock, the originally unreliable clock was accurate. Earlier, they marked the whole 24 hours of day and night on the clock face, which was later called "average solar day". In addition to the rotation of the earth, there is also the phenomenon of revolving around the sun. So is the "day" set by the rotation of the earth exactly 24 hours? The answer is no. Because the earth follows the right-handed representation, it orbits the sun once in about 365.25 days, and when the earth rotates once, it also walks about 1 degree in the orbit of revolution, so a day is slightly less than 24 hours!

3 degrees north latitude mainly refers to the range covered by 3 degrees north latitude fluctuating up and down by 5 degrees. The 3-degree north latitude line runs through four ancient civilizations and is a mysterious and strange latitude.

the clock is one of the earliest objects invented by human beings, because it is necessary to continuously measure the time interval. Some natural time intervals (such as days, leap months and years) can be obtained by observation, and shorter time intervals require the use of the clock.

The principle of thousands of years' time measuring equipment has also changed greatly. The sundial measures time by the change of the shadow of an object on a plane, and there are many kinds of instruments to calculate the time interval, including the most widely known hourglass. The water clock matched with the sundial may be the earliest timing instrument.

Europe invented the escapement in 13, and later created the first mechanical clock, which can use oscillating timing equipment such as a balance wheel. Clockwork-driven clocks appeared in the 15th century, and the clock industry began to develop in the 15th and 16th centuries, and the pendulum clock was invented in 1656.

Therefore, the accuracy of timing was further improved. At that time, the accuracy of time required by navigation also promoted the reliability and accuracy of clocks. The electronic clock was patented in 184, and the development of electronics in the twentieth century produced a clock that could completely eliminate the mechanical movement.

At present, the timing element in the clock is a harmonic oscillator, an object that oscillates at a fixed precise frequency. The harmonic oscillator may be a simple pendulum, a tuning fork, a Shi Ying crystal, or the oscillation of electrons when an atom emits microwaves.

analog clocks use hands and angles to represent time, while digital clocks use numbers to represent time. There are two time representations: 12-hour clock and 24-hour clock.

most digital clocks use electronic equipment, liquid crystal, LED and vacuum fluorescent display to display the time. Clock function is also one of the standard functions of computers and mobile phones.

for convenience, distance, telephone or the needs of the blind, an auditory clock with sound is used to tell the time. In order to meet the needs of the blind, there are also blind clocks that can sense their time by touching, some of which are similar to the traditional time, but by adjusting their design, they can directly touch the surface to know the time without affecting the timing function. Timing technology is also constantly evolving.

Extended data:

Primitive people judged time by the change of sky color and the brightness of the sun. Ancient Egypt discovered that the shadow length would change with time, and invented the sundial to measure time in the morning. They also found that the time needed for water to flow was fixed, so they invented the water clock. In ancient China, people also had a tool to measure time by water-copper pot dripping.

In China, besides using water to measure time, people in ancient China also used burning incense to measure time. Dragon boat time is more fragrant, which is an instrument that uses incense burning to time. It also has the function of ringing at regular intervals.

There are several small strings tied with metal balls at both ends on the dragon boat, and burning incense is placed under the strings. Every once in a while, the incense will burn a thread, and when the metal ball falls into the container below, it will make a chime. This kind of incense burning timepiece was first seen in the literature of Song Dynasty.

It is not accurate to calculate the time with Xiangxiang, but it was once very popular because it was simple and very suitable for folk use. According to the literature, some of them are more fragrant and can burn for a day and night, and some can even burn for a month.

in 188, Su Song and Han Gonglian, scientists of the Song Dynasty, made the first water-borne instrument stage in history, which was a combination of an armillary sphere, an elephant and a mechanical timer.

It is powered by hydraulic power and has a scientific escapement mechanism. It is about 12 meters high and 7 meters square. It is divided into three layers: the upper layer is equipped with an armillary sphere for astronomical observation; In the middle layer, you can simulate celestial bodies for synchronous demonstration; The lower layer is the heart of the instrument, and the formation and output of timing, time telling and power source are all in this layer.

in 1276, Guo Shoujing in China in the Yuan Dynasty was made into a big lamp leak. It is driven by water power, driven by a gear train and a rather complicated cam structure, to drive the puppet to automatically tell the time of "ringing the bell for one moment, engraving the drum for two minutes, ringing the cymbals for three times and ringing the cymbals for four times".

Since the Song Dynasty, the 24-hour system started at 12: , and one minute is 15 minutes today. Its accuracy is more than 3 years earlier than that of the German desk clock.

In 1283 AD, the first mechanical clock driven by weights appeared in a monastery in England.

In the 13th century, monks in northern Italy began to build bell towers (bell towers) to remind people of the time for prayer.

In 136 AD, Zhan Xiyuan created the "five-wheel hourglass", which was composed of gears and time disks.

in the 16th century, there began to be desk clocks in Germany. Those clocks have only one hand, and the clock face is divided into four parts, so that the time is accurate to the nearest fifteen minutes.

in 1657, huygens discovered that the frequency of pendulum can calculate time, and made the first pendulum clock. In 167, an Englishman William Clement invented the anchor escapement.

In 1797, American Eli Terry obtained the patent right of a clock. He is regarded as the ancestor of American watch industry.

in 184, Alexander Bain, a British watchmaker, invented the electric clock.

in 1946, American physicist Dr. isidor rabi figured out the principle of atomic clock. Two years later, the world's first atomic clock was created, and the atomic clock is still the most advanced clock. Its operation is completed by the natural vibration of cesium and hydrogen atoms, and it can operate accurately within 3 years with very small error.

from 18th to 19th century, watch manufacturing industry gradually implemented industrial production.

in the 2th century, it began to enter the period of Shi Ying.

in the 21st century, the technology of radio-controlled clocks which can automatically adjust the time according to the principle of atomic clocks has gradually matured.