The reasons for the site selection of Zhougong observation deck

Why did Duke Zhou come to Gaocheng to measure the sun shadow? We know that "success" means "ancient Yangcheng". The book Shuo Song pointed out: "Gao Cheng is the master of Guyang City." Mencius also talked about "the son of Shun in Yangcheng, Yu Shun". Wu Zetian arrived in Yangcheng after the Dengfeng ceremony in Songshan. To celebrate this achievement, she changed Yangcheng into a success. And why did Duke Zhou go to Chenggong to measure the shadow? In the book Astronomical Monuments in China, edited by Xu, it is pointed out that the reason why Zhou wanted to go to Yangcheng to measure the sun shadow is because Haojiang, the capital of the Western Zhou Dynasty, is located in a remote place with inconvenient transportation, and Yangcheng is located in the Central Plains with rich products and developed culture, so Duke Zhou wants to move the capital to the Central Plains. When astrology is popular, it is necessary to make some sense when moving the capital. The rulers of the Zhou Dynasty created public opinion that Yangcheng was located in the "Central Plains", which meant the center of the nine continents in the world. "Zhou Li" has such a record: "The scene from the sun to the sun is five inches long, which means that on the land, heaven and earth meet, the four seasons meet, the wind and rain meet, and the yin and yang meet, but it is foolproof. This is to establish a kingdom." As we know, at the same latitude as Yangcheng, the shadow of the eight-foot table is one foot and five inches on the summer solstice, which is called "the ground" and actually refers to the north-south midline of the country at that time. According to the county records, the platform built by Duke Zhou when he built Luoyang as the capital of the East was used for "measuring the depth of the soil, observing the sun, seeking the land and measuring the four seasons". In other words, we can measure the earth by standing on the earth and measuring the shadow of the sun, and test the seasonal changes of the four seasons. The principle and method of measurement are as follows: 1. The GUI is perpendicular to the desktop (that is, 90 degrees); Second, the gauge setting must be consistent with the local meridian (that is, south and true north); Third, to observe the sun shadow, we must measure it every day, day after day, and record the data of the length of the sun shadow measured every day one by one, and find out the seasonal changes according to the daily changes of the sun shadow. Duke of Zhou designated the day with the longest shadow as the "winter solstice". At noon on this day, the sun shines directly on the Tropic of Capricorn, and the daytime in the northern hemisphere is the shortest; The day with the shortest surface shadow is designated as "summer solstice": the sun shines directly on the Tropic of Cancer, and the day in the northern hemisphere is the longest. The longest day in one year and the cycle of the longest day in the next year, that is, the earth goes around the sun, are designated as the "tropic year" There are two days in a year with the same appearance and the same day and night time, which are designated as "vernal equinox" and "autumn equinox" respectively, and then 24 solar terms are gradually summed up to serve human production and life.