2. The reason for the emergence of 40 days is that the initial lodging is generally 10 day, the middle lodging is 10 day or 20 days, and the last lodging is 10 day. The annual lodging time in dog days needs to be calculated by consulting the almanac. According to the ancient popular trunk and branch calendar method, there are 10 days and/kl.

3. The annual lodging time is counted from the third Geng day of the summer solstice, which is repeated every 10 day. If there are four Geng days between the summer solstice and beginning of autumn, the mid-lodging is 10 day. If there are five Geng days, the mid-lodging is 20 days, and the annual Geng day is uncertain, so the dog days are sometimes 30 days and sometimes 40 days.