In ancient Chinese, "Duan" has three meanings: beginning, right and extreme; "Wu" and "Wu" have the meanings of "Zhong, Zheng and Ji", that is to say, "Dragon Boat Festival" was originally a concept of astronomy and astronomical mathematics. As we all know, the summer calendar in China is October in the solar calendar, that is, a solar year is counted as ten months. In the basic count of "ten", five is the median, which indicates the number of regression digits in a solar year, and some of them are similar to the number of "dead points" in circular motion and "inflection points" in curves in physics. Since the implementation of the "taichu calendar Law" in the Western Han Dynasty, the two solar terms, winter solstice and summer solstice, respectively represent the position of the sun on the Tropic of Cancer. That is to say, in a year, the winter solstice in the summer calendar (about 65438+February 2 1~22) is the festival at the extreme position of the tropic of Capricorn, while the summer solstice (June 2 1~22) is the festival at the extreme position of the tropic of Capricorn.

The solstice in winter is the day that China people feel has the shortest day and the longest night in a year. Every year after the winter solstice, the sun begins to shift from south to north relative to the earth, so the day begins to get longer and the night becomes shorter on the basis of the winter solstice. In contrast, the sun gradually moves to the position of "summer solstice" relative to the earth, which is the longest day and the shortest night felt by China people; After the summer solstice, the sun began to turn from north to south relative to the earth, so under the reference system of summer solstice, the days began to get shorter and the nights began to get longer.

What is the position of the fifth day of May in the summer calendar in the sky? This is a festival close to and around the summer solstice, or a festival between "Miscanthus" and "Summer solstice". For example, the annual Dragon Boat Festival falls on May 5th in the summer calendar, and most of its festivals are sandwiched between15th and "Summer Solstice Day", sometimes it falls within a few days after the summer solstice. In other words, the so-called "Dragon Boat Festival" actually refers to the day when the sun approaches the Tropic of Cancer, and then, compared with the summer solstice, the days will start to shorten and the nights will start to lengthen. However, the day of geographical heat storage is coming, so it will be hotter after the Dragon Boat Festival.

To sum up, we know that "Dragon Boat Festival" means "a festival close to the longest day in a year". Knowing the astronomical significance of the Dragon Boat Festival mentioned above, we should understand why the ancient Chinese called it "Dragon Boat Festival, Duanyang, Noon, Midday and Forever".