We know that certain alignments are astronomically aligned, particularly toward the rising and setting points of the sun at the solstices, or toward the rising or setting points of the moon at lunar maxima or minima. But research has not yet been sufficiently systematic to identify how many of these there are, and the patterns of their application.
There are two distinct issues here. One is the rising or setting points of sun and moon on the local horizon, as seen from an ancient site. That horizon might be higher or lower in elevation (height) when observed from the place of observation, which is called the backsight - often it's higher more than lower. This means that, at that place, the sun or moon will rise at a different degree or azimuth from the place where it would rise if the horizon were level.
The other issue concerns a level-horizon azimuth, which holds true whatever elevation the local horizon is. Unless one is looking over the sea or across a relatively flat landscape, the level-horizon azimuth is a theoretical more than an observational azimuth.
If you look at Professor Thom's diagram below, the double circle is the sun rising over the sea - a level horizon (if you're standing at sea level, not on top of high cliffs). The lines above the sea horizon represent hill-horizons of different heights, up to two degrees higher in elevation, as seen from where you're standing. As you can see, the place where the sun would rise would move further to the right, or south, the higher that the hill horizon is. That left-right motion is the azimuth.
So the two big variables here are the azimuth and the elevation. The azimuth is the number of degrees from true north, measured eastwards or rightwards from north along the horizon.
So northeast is 45°, east is 90°, southeast is 135° (90+45°), south is 180°. Southwest is 225°, west is 270° and northwest is 315°. North is at 360°, usually called 0° and the degree immediately to the left of it is 359°.
The elevation is the vertical height of any point on the horizon (the foresight) relative to the place of observation, where you're standing (the backsight). Usually this is just one or two degrees up or down, with distant hills, but it makes a difference - as is demonstrated in the diagram above, showing the influence of the horizon on the rising point of the sun at solstice, at the latitude of 50° north, the latitude of West Penwith. But in some cases, at sites in valleys (such as Mên an Tol), the horizon can be quite high (in this case, at Mên an Tol, looking eastwards, 30ish° or more).
So, if you were building a chambered cairn or a stone circle, and you wished to make an orientation to the summer solstice sunrise at that place, you would observe it at the site and at that time, but you could also calculate it mathematically if you know the elevation and the level-horizon azimuth of sunrise at summer solstice. In Penwith it is 64° east or rightwards of north.
If you stand on the top of Trencrom Hill for summer solstice sunrise, it will rise over St Agnes Beacon, miles up the coast - magic! Solstice-oriented hills!
Near-total solar eclipse, 20th March 2015, from the Botrea Barrows
The ancients of the megalithic period were obsessed with time - rather like the Mayans, two millennia later in Mexico. They were preoccupied not just with counting time, as we do when we nowadays say "4pm on 15th June 2005", but with understanding the nature and the passage of time - in the sense of 'changing times'. They wanted to understand the nature of the times, to plan their activities accordingly. Having a calendar was important, but there was much more to it than this. It went right to the heart of their beliefs about life, the universe and everything.
They sought to play an active role in the movement of time and season. They felt their fortunes were closely connected with the fluctuations of Time and their relationship with it - how they responded to it, what they did with it, what they gained and what they could contribute. Time is the medium through which Great Spirit, the Awen of the Druids (what Christians, Muslims and Jews call 'God'), manifests itself into the physical, through the agency of ever-changing times.
They wanted to do their best with the available qualities of Time, and to foresee coming times that were auspicious (such as solstices or eclipses) or inauspicious. The relationship between the heavens, humanity and the earth needed to be in harmony and order, and if this was so, all would be well and everything and everyone would flourish - in this they were rather like the Taoists of ancient China. They sought to build time and its cycles into the location and design of the ancient sites they built. Thus they were fixing time into the dimension of space or landscape.
Time was fixed into space by creating astronomical alignments and orientations to the rising and setting points of sun, moon and possibly the stars (though proving the relevance of stars in this context is difficult, and could only properly be resolved by speaking to one of the original megalith builders). It was also fixed through the number of stones in stone circles and their mathematics. This would in effect dedicate a site to marking one or a few specific times of year or of the nineteen-year lunar eclipse cycle.
A key period was the Metonic cycle, named after a Greek philosopher, Meton, who was alleged to have discovered it - though he did so 2,000 years later than the stone circle building period, so he doesn't really deserve the credit, but he was probably the first to put it down in writing.
The ancients actually had two parallel calendars - one solar and one defined by the phases of the moon. The solar calendar was mainly an eightfold calendar, not twelvefold - the twelvefold calendar was invented by urbanised people in early urban civilisations such as Sumer. The lunar calendar, measured from newmoon to newmoon, was 29 days and 12 hours long.
The Metonic cycle brings together the solar and lunar calendars, which coincide once every 19 years - specifically 6,939.6 days. This is a cycle where various sun-moon cycles coincide. It is equivalent to 19 solar years, 20 eclipse years (cycles of eclipses), 235 synodic lunar months (cycles of lunar phases), 254 sidereal lunar months (lunar orbits of the heavens), and 255 draconic months (conjunctions of the Sun with the North Node or Dragon's Head - an eclipse-related cycle). (This is also explained on the stone circles page.)
In Penwith it is necessary to find out which sites and alignments have astronomical orientations, and also which do not. The azimuth orientations of all alignments between sites in Penwith have been calculated (as if on a flat landscape), and this is presented below in a downloadable PDF file. At a later date, site orientations will hopefully be studied more comprehensively in the field. This has been started particularly by Caroline Kennett - see Archaeoastronomy Cornwall.
Calculating and listing alignment azimuths does not mean that we suggest many or all of the alignments are astronomical - it is simply a means of finding out and studying the matter more closely. More work needs doing on this.
The problem with studying the azimuths of sacred site alignments is that it presumes a flat landscape and horizon which, in many cases, is not the case. If one follows an alignment, the physical horizon ahead will change in elevation. So astronomical alignments are probably taken only from one major site on that alignment, and progressed from there.
However, you will notice in the list below that there are concentrations of alignments around certain azimuth degrees. This is either a matter of chance, or it is not - we will not know until we have carried out a full archaeoastronomical field survey.
You can view the azimuth list below or download it as a PDF file here.
So, although we do know of a number of orientations of sites and alignments to and from them that seem to be astronomically aligned, this has not been studied sufficiently. In some cases, an orientation that is reckoned to be to a certain point, such as that of the winter solstice sunrise or a lunar maximum, it has not been sufficiently checked. There are also problems with hedges, trees and other more recent features in the landscape such as buildings, which can make field observation difficult (for example, at the Merry Maidens complex).
But hopefully a systematic survey of significant sites (perhaps 40-50 out of the 500ish sites in Penwith) can one day be done - it's all a matter of (mainly) time, energy, organisation and expense (mainly petrol and sandwiches)!