Date: Fri, 6 Feb 1998
From: Simon Cassidy <scassidy@EARTHLINK.NET>
Subject: Re: Stonehenge et al (75-stone model)
Lance Latham asked Simon (re Stonehenge central sarsen structure):
> Can you briefly describe the significance of the 73-stone model, as
> opposed to the 75-stone model? The idea of relating Stonehenge to
> other calendric/astronomical structures in other cultures at similar
> stages of cultural development is a valid one, I believe.
The 75-stone model (dominant assumption) consists of 30 circle uprights 10 trilithon uprights, 30 circle lintels and 5 large trilithon lintels.
If the circle represents a lunar month (say the usual number of days between full moons) then the horseshoe of trilithons can count safe full moons (i.e. never eclipsed) between full-moons endangered by a lunar eclipse. I.e. if the gap in the horseshoe represents (by absence of stone) an unsafe full-moon then the five trilithons can represent the usual five safe full-moons before another eclipse endangered full-moon.
Exceptions to this usual six-month cycle are represented by the occasional need to count each trilithon upright as a safe full-moon, giving a count of ten safe full-moons between certain eclipse-endangered full-moons.
This significance also applies to the 73-stone model but the 75-stone model differs from the 73-stone model if use is made of the lintel numbers to prescribe the frequency of the ten-moon eclipse-dearths amongst the usual five-moon eclipse dearths.
The 75-stone model can prescribe a frequency of 5 (the number of trilithon lintels) ten-moon sequences, amongst 30 (the number of smaller lintels around the circle) five-moon sequences. This gives a complete eclipse cycle (including the eclipse endangered full-moons) of 235 full-moons. We know this cycle as the nineteen-year Metonic cycle, but use is made here of its lesser known property of also being a fairly good short-term eclipse cycle (38 to 57 years in duration).
This about sums up the calendrical significances of a 75-stone central sarsen structure, other than to point out the reemphasis of these features in the subsequent (somewhat architecturally degenerate) additions to the structure over the ensuing millennium (2000-1000 BC). As I pointed out previously in my response to Kevin, all the sets of bluestones and pits added later can be seen as repetitions of the counts of days in one or two lunar months. The final total of ground positions looks like a count of 6 lunar months (the usual eclipse cycle of 5 safe full-moons followed by an eclipse endangered full-moon) with the set of circles (i.e. the non-horseshoe elements) counting 5 lunar months; (another way of occasionally correcting the usual 6-moon cycle is to allow for an endangered full-moon after four safe ones, rather than ten safe ones). A 235 full-moon cycle (of nineteen solar years) can be seen to be reemphasised by the nineteen bluestones later erected, in a horseshoe inside the five sarsen trilithons.
This seems like a complete and comprehensive calendrical interpretation of Stonehenge phase III (2400BC-1000BC) placing emphasis on lunar phase phenomena (especially lunar eclipses), with just a nod towards the sun in the form of the "Metonic" nineteen-year cycle of lunar phenomena.
Hoyle seems not to have been satisfied with this, and suggested (in his book "On Stonehenge") that the nineteen bluestones could have represented, not just years, but eclipse-years (of 11 or 12 lunar months each) and thus the bluestone horseshoe could represent not only a "Metonic" cycle (of 235 phase-months) but also a "Saros" cycle (of 223 phase-months) which would greatly enhance the accuracy of numerical lunar-eclipse prediction.
This (the greater utility of the "Saros" cycle for eclipse prediction) and the examination of similar numerical developments in Babylonian and Mayan astronomy lead directly to a consideration of the 73-stone sarsen model. But I will leave this consideration to a follow-up message.