Brief introduction of Shang yuan chronology

In ancient calendars, the era was generally set as the starting point of calculation. This starting point is usually an ideal moment. It is usually based on midnight of Jiazi Day, which is the new moon and the winter solstice solar terms. Push up from the era, and find a moment when the astronomical phenomena of "the sun and the moon meet and the five stars meet the pearl" appear. This day is exactly the same as the latitude and longitude of the moon, and the five planets gather in the same direction. This moment is called Shangyuan. The number of years from Shangyuan to calendar year is called cumulative year, commonly known as Shangyuan cumulative year. Shangyuan is actually the same starting point of several astronomical cycles. With Shangyuan and Shangyuan cumulative years, it is more convenient for calendar experts to calculate the movements and positions of the sun, the moon and the five stars.