How are the dog days stipulated every year?

Three volts are divided into initial, middle and final volts. The dog days are not exactly the same, with 30 days and 40 days.

The specific date of the dog days is determined by the solar term date and the dry branch date. According to the traditional calculation method in China, the third and fourth Geng days from summer to the future are the start dates of the first day (recumbent) and the second day (recumbent) respectively, and the first Geng day after beginning of autumn is the first day of the last day (recumbent). Gengri is an auspicious day with the word "Geng" in it, such as Gengzi and Gengchou ... Such dates are published in the yellow calendar every year.

Because the third and fourth Geng days are the first days of initial and intermediate lodging, and the interval between each Geng day is 10 day, the initial lodging time is 10 day, and the final lodging rule is 10 day. There are long and short squats, which may be 10 days or 20 days. Every year, the third Geng day after the summer solstice solar term appears in the morning and evening, so the number of days in the middle is different, so some years are 30 days apart, and some years are 40 days apart.