A subscriber asks:

I noticed that tomorrow’s (6/16) lunar hexagram jumps from Shi [7 The Army] to Gou [44 Pairing] even though today it is only line 5 of Shi.  Shouldn’t tomorrow’s hexagram still be Shi, Line 6? 

This is a good question, and I’m glad you noticed the apparent anomaly!  

The answer is that the Lunar Hexagram cycle is keyed to the arrival of each new moon.  For Moons that last 30 days in the calendar, all of the five Hexagrams included in that Moon have time to go through all 6 Lines.  For Moons that last only 29 days, however,  the last Line is skipped in order for the next day to start with the proper first Hexagram for that Moon, beginning with its Line 1.  

The length of each Moon is governed by astronomical timing relative to the date in China on which it happens.  Consequently, there will be some years when (for example) the Snake Moon will have 30 days, and the last line of Hexagram 7 will be included, and other years (like this one) when it will only have 29, and Line 6 will be skipped.

Please let me know if you have further questions I can address, and thanks for being a subscriber to Ming Li Tong Shu Online!