The Calendar is incised on one of the kerbstones of the Knowth mound: 'a luni-solar time measuring device designed and engraved c 3,000 B.C.' Martin Brennan explains it thus:

The symbols on the Calendar Stone convey three series of observations of the passage of time, based on the perpetual progression of the months and regulated by the phases of the moon. Each series of symbols in the design returns to its starting point, and all three series are related to each other, providing extensions and regulations of the basic information.

The first cycle or series of symbols, numbered 1 to 27 above, show the phases of the moon daily through the month. The waxing moon, 1, emerges from behind the spiral to begin its path through the sky. By position 10 the crescent shape has changed to an oval which then becomes a circle during the period of the full moon, 14-16. The waning period shows the return of the crescent shape which, at day 27, returns again behind the central spiral. This is the last glimpse of the visible crescent on the eastern horizon. The monthly cycle repeats with the emergence of the crescent again at position 1. Thus, a lunar month of 29 or 30 days is accurately described.

The second cycle is shown in the figure incised inside the first series. This consists of a wavy line on which each movement, up or down, represents one lunar month, or one complete cycle of the first series. The equinoxes vernal and autumnal, are indicated in the figure by the letters V and A respectively, recurring at six-monthly intervals. Reading from the vernal equinox at the left of the figure, the period represented across the figure is thirty-one lunations, the thirty first being the correcting factor to reconcile lunar with solar time (2^ solar years - 31 lunar months). The count continues in reverse from right to left and the further correcting factor of the intercalation of a lunar month before the cycle recommences is shown by the figure at the left end of the design, marked by the dot. The equinoxes fall in the same solar months in either direction on the design.

The third cycle, represented by the device at the bottom right of the design, covers a much greater time span. Each thirty-one lunation movement from the second cycle is represented by one up or down movement on this figure (numbered 1-8). This shows the great cycle of nineteen solar years or 235 lunations after which the next vernal equinox begins a new cycle.

What mysteries do these stones still hold for us?

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