The 13 Moon calendar is based on the 28-day harmonic lunar cycle. This is the amount of time it takes the Moon to orbit the Earth, when seen from outer space. During this time it turns on its axis but once, which is why we are always seeing the same side of the Moon. The Moon turns 13 times on its axis in the time the Earth rotates once around the Sun, hence 13 Moons = one year.
Commonly used lunar phase calendars are actually measures of the Moon’s synodic cycle, from new moon to new moon as seen from Earth, which is a cycle of 29.5 days, or twelve lunar months for a lunar year of 354 days.
The other lunar measure as seen from Earth, is the sidereal cycle, the measure of the time it takes the Moon to return to the same place in the sky. The sidereal measure is 27.3 days. By comparison there are 13 cycles of the sidereal measure to 12 of the synodic.
The 28-day measure is the harmonic standard that most accurately matches the Moon’s actual cycle of rotation. As a product of 4 x 7, the 28-day cycle also accounts for the measure of the four phases of the moon.
The 13-Moon calendar reflects the annual human biological rhythm of the woman’s 28 day fertility cycles.
Indigenous people also share the knowledge of the thirteen moons on a turtle’s back. As children some are taught to examine the segments on the back of every turtle. There are thirteen large segments which represent the thirteen moons which make up the lunar year.
By counting the smaller segments around the lower edge of the shell, you will find there are 28, which represents the 28 days between new moons.