A Stonehenge Calendar
There are many coincidences between Stonehenge and the Celtic Tree Alphabet and, whilst Stonehenge's mathematics and geodetics are well-documented elsewhere* and a thousand years earlier than the Alphabet, the relation between the two is worth exploring .
The clock
Stonehenge's 'clock' works in similar principle to a contemporary clock. The fifty-six Aubrey postholes equate to the second hand and represent a month, the thirty stones of the Sarsen ring equate to the minute hand and represent a year, whilst the nineteen inner stones of the Bluestone horseshoe form the hour hand and represent the 'Golden Year' (or Metonic cycle) of nineteen years.
Working
inwards
At the outer edge, each of the 56 Aubrey postholes represents a day or a night, hence the 28 'day' (lunar) month, (the solar or 'Egyptian' month had 30 days like the sarsen ring). At the centre, within the five paired trilithon 'vowels', lies the 'Golden Year' of the 19 stones that completes the inner 'alphabet' horseshoe, and defines the completion of both lunar and solar calendars.
The Lunation Triangle
A core mechanism of the Stonehenge clock is the 'Lunation triangle', defined within the Station rectangle by its 5:12:13 proportions. Whilst well-known for its geodetic importance*, the 13 and the 12 are also both expressions of months. The lower 12 are solar (or 'Egyptian') months, while the upper 13 are lunar (or 'Celtic') months. Because the 5 lies at a right angle to the 12, it automatically creates the (pythagorean) triangle that harmonises it to the 13, and shows how the 5 alphabetical 'vowel' points relate to the 13 'consonant' periods.
* see Robin Heath Stonehenge in particular
** see John Michell's work