While the Lunation ring is a representation of the lunar synodic month, the moon ring is a representation of the lunar sidereal month which is approximately 2.2 days shorter. This is the time it takes the moon to orbit the earth relative to the zodiac. The following diagram illustrates the difference between the lunar sidereal and synodic months.
The moon, sun and node rings are based on the 56 Aubrey holes at Stonehenge. The moon ring could have only 28 holes, but by doubling this to 56 holes we can increase the accuracy of the calendar and enable us to make corrections because of the discrepancy between 27.3 days and 28 days.
If we were to choose to use the 28 days it takes for the moon to orbit the earth as the length of a month, then 13 months would be 364 days long. One day shorter than the solar 365 day year, making a full year from a lunar perspective a year and a day, a phrase very familiar from Celtic tradition. This system is advocated by Robert Graves among others, however, due to the fact that the sidereal month is actually 27.3 days (0.7 days shorter), and the solar year is 365.25, this system accumulates an unacceptable discrepancy of 10.35 days per year.
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