Chapter 61 Section 3 Dai Calendar

More than 2,000 years ago, the Dai people had exchanges with the Han people in the Central Plains and were influenced by the Han culture.In history books, it is called "Yongchang's outer Shan country" and "Rinan's outer Shan country".At the same time, the Dai people were influenced by Theravada Buddhism from India and absorbed some content of the Indian calendar, which formed a unique style of the Dai calendar. The Dai calendar era is set on March 22, 638 AD.There are 12 months in a normal year, with 30 days in a single month and 29 days in a double month, a total of 354 days.August is a double month, which should be the 29th, but every few years, one day will be added to become a big month.The Dai calendar stipulates that the synodic month is 29.530583 days, and the practice of making the month larger in August is actually to adjust the length of the month by leap day to make it close to the synodic month.The Dai calendar not only has leap days, but also leap months.The leap month is fixed at 30 days, so there are 384 or 385 days in a leap year.The leap cycle adopts the 19-year seven-leap method, and the leap month is stipulated to be set after September, which is called "leap September".Therefore, the Dai calendar is a lunar calendar.

It is customary for the Dai people to divide each month into upper and lower parts, with 15 days in the first half of the month and 14 or 15 days in the second half of the month.The first half of the month starts from the first day, called the first day of the moon, the second day of the moon, until the 14th of the moon, and the last day of the first half of the month is the 15th of the moon.The second half of the month starts from the first day, which is called the first day of the next month, the second day of the next month, until the 14th day of the next month, and the day at the end of the month is called the day of the black moon.Because the Dai calendar pays more attention to hope than Shuo, and uses Pingshuo, the first day of the Dai calendar and the first day of the Han calendar often do not coincide.

In the Dai calendar, the twenty-seven constellations and the twelve palaces are used together to divide the sky.When calculating the positions of the sun, moon and planets, it is expressed in the ecliptic coordinate system. The ecliptic circle is 360°, and 1° is equal to 60′.The sidereal periods of the planets in the Dai calendar are very precise, and the values ​​of Venus and Mercury are consistent with the modern theoretical values. In the past, the knowledge of astronomy and calendar could only be mastered by Buddhists among the Dai people, and it was the privilege of Buddhist temples to compile and print almanacs (see Figure 24).