While playing in a campaign of Werewolf: The Forsaken, I volunteered to make an Auspice Calendar. Our group was having a hard time figuring out which auspice was currently active and how long that phase of the moon should last. I decided to use a real lunar calendar and began plotting out the days of the New Moon throughout the year to start.
During my first attempt at creating an Auspice Calendar I chose a 28 day cycle. Unfortunately, this very easy method does not work – the cycles did not line up with the real life new moon phases. The real moon orbits the earth in of 27.3 days, but because of the Earth’s orbit around the sun, it appears to earthbound observers to take 29.5 days to complete a cycle from New Moon to New Moon. A 28 day cycle, as suggested by others (5 days of New Moon & Full Moon every month with 6 days of each other auspice) does not provide a remotely accurate calendar. Using a 28 day calendar would result in a homogeneous calendar where every single full moon lands on the same day of the throughout the year, every single new moon lands on the same day of the week and so on.
A 30 day calendar (with 6 days of each Auspice) is also inaccurate. By plotting the actual dates of the New Moon on a calendar, a 30 day system of Auspices quickly falls off track from the real new moon phases. Since every lunar month is actually a half day short of 30 days, the cycles are not perfect.
In order to provide an accurate lunar calendar, the actual dates of the year’s new moons were marked the calendar. Each lunar cycle contains 5-6 day of New moon and Full Moon phases. On months where the lunar cycle only contains 29 days, either the New Moon or the Full Moon phase was shorted a day. The phase that would be shorted a day was rotated evenly. In the end, both the New Moon and Full Moon phases lose 3 days in comparison to the other phases.