Good Source - Austin Coppock’s book on the subject
The visible lunar cycle of 29.3 days does not fit into the 365.25 days of the solar year. Much to the chagrin of calendar crafters, the lunar cycle falls nearly 10 days short. To bridge the gap between the solar year and the lunar month, a number of cultures, among them the Chinese, Arabian and Hindu, utilized the intercalary or “leap” months, an awkward system which added an extra month to the calendar every three years. In many cultures, the Sun’s cycle was first measured at the periphery of the historical record. For the Egyptians, it was sometime in the late 3rd millennia. This is where the decans first appear, as a division of the year into 36 distinct sections, each with its own associated characteristics and gods. Yet what is interesting about the decanic division into 36 is that it addresses not just the year, the large wheel of solar time, but also the day—the small wheel whose rotation we are constantly privy to. Each of these cycles is cut to 36 by means of marker stars. The rising of each one of these stars in the night sky marked the beginning of a new hour. The decans, and the stars which marked them, were thus a type of stellar clock, allowing the ancient Egyptians to divide the night. This is a significant innovation, for the observant have long used the angle and length of shadows to measure the day, but night offers no such conveniences.
- Austin Coppock