What is the purpose of vertically inserting a pole on the ancient ground?

It's easy to understand what it feels like to measure shadows. In most parts of our country, every sunny day at noon, all kinds of objects have a shadow to the north. Of course, this is no exception for telephone poles, which also have a clear shadow on the ground. If this pole does not move for many years, and you often go to see the shadow at noon, then you will find that the length of the pole shadow always changes all year round: summer is very short; Winter is very long. If you look more carefully, you will also find that one day the shadow of the pole is the shortest in summer and the longest in winter. These two days, our ancestors gave them a name called "Solstice". The record of solstice already exists in Oracle Bone Inscriptions. In the Spring and Autumn Period, it was very common to use this method to determine the "solar solstice". At that time, an ancient book named "Zhou Li" said: "Measure the depth of the soil by the method of soil gauge, and then look at the sun, so as to enter the ground. In the south, the scenery is short and hot; In the north, the scenery is long and cold. " The "earth gauge" mentioned here is a ruler carved from jade. Putting it on the ground can measure the length of the polar shadow. The "scene" here is the word "shadow" now. The general idea of this passage is that the center of the earth can be determined by measuring the noon sun shadow with soil gauge. In summer, the sun shadow is short, and the day is in the south. In cold weather, the sun shadow is long and the day is in the north. In this way, the solar solstice was divided into two in ancient times: one was called the solar solstice or the summer solstice; One is called the northern solstice or the winter solstice. But when did the summer solstice and the winter solstice happen? Another ancient book reflecting the Spring and Autumn Period, The Sutra of Zhou Bi-suan, clearly records: "The Sutra of Zhou Bi-suan is eight feet long, and the sundial is one foot six inches in summer and three feet five inches in winter." The "circumferential tilt" here is the pole or pillar mentioned above; Qi is the shadow of the sun. This pillar stands in Dengfeng county, Henan province, and its remains still exist. This passage means to erect an 8-foot pole. If the shadow of the pole at noon is 1' 6 ",then this day is the summer solstice; If the shadow length is 1.3 feet 5 inches, then this day is the winter solstice. Long ago, our ancestors knew the length of a year in this simple and scientific way-from summer solstice to summer solstice, or from winter solstice to winter solstice.

There is a saying in Shangshu that records the ancient history of China: "There are six days in 360 years, and the leap month is set at four o'clock." This shows that the length of a year in ancient times was 366 days. Later, when I came to Zhou Kuai Shujing, I had a new understanding. There is a passage in the book that says: "So the shadow of the South Pole is long in 365 days, but it will be short tomorrow, and it will be long all year round, so we know that there are three in 365 days and one in 366 days." So, I know that a year is 365 days and one quarter, and a year is over. "That is to say, the length of 1 year is determined by the method of measuring the shadow of the week. In the observation of four consecutive years, the age of three years is 365 days, while the age of 1 year is 366 days. So on average, the length of a year is 365 and a quarter of a day. This number plays an extremely important role in the calendar, and we will often mention it in future introductions.

The establishment of the concept of "year" has made mankind take a decisive step in timing, and a longer time unit "year" has appeared on the basis of "month". At this point, the basic elements of the calendar-year, month and day-are all ready, and various calendars have developed under this condition and become more and more accurate.

With the progress of the times, the original soil gauge has been replaced by a more exquisite and scientific astronomical measuring instrument-"gauge table". This instrument consists of two parts: a vertical copper column or stone pillar, called a "watch"; Another kind of flat copper ruler or stone ruler is called "Gui". Put the ruler in the north-south direction, and install a vertical table at the southern end of the ruler, so that you can use it to calculate the occurrence time of the four seasons and solar terms through photogrammetry. Since the Qin and Han Dynasties, China has made this kind of standard watch. There are still two preserved ones: one was made in the second year of the Ming Dynasty (AD 1437) and is now in the Purple Mountain Observatory in Nanjing; The other is a small bronze watch unearthed in Jiangsu during the Eastern Han Dynasty. The watch is connected with Guiyu through a shaft. It is upright when it is used and flat when it is not used. The total length is only 34.5 cm, which is very convenient to carry. This is a very precious ancient cultural relic.