This is a picture taken from the book Yuan Hai Zi Ping 淵海子平 written by Xu Sheng 徐升 during the Song Dynasty 宋朝. The diagram shows the instruments used by an astrologer using the Zi Ping method. The water containers were designed to allow water to drip so that the water level showed the time at night. During the day, the length of the shadow of the pole cast by the sun showed the time.
Zi Ping method is the study of qi indicated by the year, month, day and hour each represented by the 60 stem-branch cycle. The 24 Jie-Qi 節氣 are used to define the months. The year begins at Li Chun 立春 and each subsequent month begins at a Jie. The entire set of four pillars are calculated using the solar calendar. The local solar time is used for the hour.
A picture is worth a thousand words.