The solar day, the solar year, and the lunar month, or lunation, may therefore be called the natural divisions of time.
The method is based upon consideration of the degree of the previous syzygy (lunation).
It must happen sometimes that in the course of a lunation the sun enters into no new sign; in this case the month is intercalary, and is called by the same name as the preceding month.
But the fourteenth of this moon falls at the latest on the 18th of April, or 29 days after the 20th of March; for by reason of the double epact that occurs at the 4th and 5th of April, this lunation has only 29 days.
The Calendar Is Constructed On The Assumptions That The Mean Lunation Is 29 Days 12 Hours 44 Min.