Seed
Last but not least - the Lunar Phases - a great third-dimensional presentation of the harmonic cycles of time. The 28-day cycle which is the base of the 13 Moon calendar is both Galactic Standard Time, and the mean lunar cycle:

Serpent
the average between the moon's synodic cycle of 29.5 days and the sidereal cycle of just over 27 days. The synodic cycle is the cycle of the moon's phases (new moon, full moon, waning, etc.)

Worldbridger
The sidereal cycle is the measure taken from where the moon appears at the same place in the sky. Look at how the lunar phases are shown in your Gregorian calendar, and compare it with how they appear in the 13 Moon calendar -