There can be no doubt of human presence upon the Isle of Avalon from a very long time ago indeed. The challenge is to discover what those ancient people thought and felt about the highly unusual island in the ‘Summerlands’ from the little evidence that remains. Although we know many prehistoric peoples dwelt around the Somerset Levels, taking advantage of the extensive resources of its marshes, we have nothing to show that they lived or farmed upon the island peninsular itself. This absence of everyday evidence, when there was so much prehistoric activity all around, marks the Isle of Avalon out as a place set apart.
The evidence that does remain, such as the great ditch and bank known as Ponter’s Ball that separates the original island from the mainland, is enigmatic and undated. When this huge ditch was full of water, Avalon became an island. A sacred herd could have been contained within the temenos it defined, and a rule to build with nothing but the wood and reeds of the marshes may have prevailed within its borders. The age of the terraces on Glastonbury Tor is also entirely unknown; while some terraces are historically recorded agricultural lynchets, others clearly are not. And no archaeologist has ever dug into the artificial mound that lies upon the summit of Glastonbury’s fourth most prominent hill (after Wearyall Hill, Chalice Hill and the Tor): Windmill or St Edmund’s Hill.
Our observation of the skies has now shown that the Mound on Windmill Hill was deliberately located to observe the Winter Solstice sunrise over the Tor. When seen from the Mound, the southernmost, lowest appearance and subsequent ascent of the Sun is precisely defined by the steep northern flank of the Tor. Today the midwinter Sun rolls up the side of the Tor, but in approximately 3000 BCE, only the top rim of the Sun would have been visible in the notches created by the terraces. The calendrical precision the terraces provided would have allowed for accurate observation of the all-important Winter Solstice from the Mound—without blinding the observers.
When seen from the St Edmund's Hill Mound, the Sun at Winter Solstice not only rises up the Tor and reaches its zenith over Chalice Hill, it also sets over the western end of Wearyall Hill. Thus the Mound provided the viewing platform to look over the entire island and relate it to the sky. Prehistoric observers on the Mound used other points on the Tor, and many points on the horizon (some natural, some human enhanced), to observe the solar and lunar extremes. They may also have placed wooden posts along the back of the Tor to help them watch the movement of the stars relative to the Sun in the slow rotation known as the precession of the equinoxes. They could have forecast the event towards the end of the fourth millennium BCE, when the heliacal rising and setting points of the brightest star in the sky, Sirius, were the same as those of the midwinter Sun. Most remarkable of all was that the entire Milky Way was visible in their era; providing a brilliant ring of starlight around the horizon. See The Star Temple of Avalon
The Mound may have been built very early on in the sequence 
of British mound building; perhaps as early as the fourth or even the fifth millennium BCE. It is more likely be c. 2000 The natural features of Avalon, its prominent hills set in a level plain, coupled with the Mound, would have assisted in developing calendrical and astronomical practices within the prevailing social, mythic and spiritual context. The island must have hummed with the life and activities appropriate to a Holy Isle – an Isle of Earth certainly, but also an Isle of the Sky.
The Situation Today
The Mound still functions today: clicking off the solstices, marking the lunar extremes, indicating the great procession of the stars through the heavens, but, perhaps most significantly, over the next few decades the Mound will continue to indicate the co-incidence of the Solstitial Sun’s risings and settings on the crossing points of the Ecliptic and the Milky Way.
Between 1980 and 2020 CE the solstitial Sun passes over the two crossing points of the Ecliptic and the Milky Way. The solstitial Sun does this approximately every 13,000 years in the slow rotational movement known as the Precession of the Equinoxes, but the current event when the Winter Solstice Sun is upon the Sagittarius / Scorpius crossing point (that also points to the heart of our galaxy, the Milky Way) and the Summer Solstice Sun is upon the Gemini / Taurus crossing or portal (pointing away from our galaxy) will not repeat for 26,000 years. The Winter Solstice Sun will be close to the galactic core for the next few decades. Thus an observer on the Mound at the Winter Solstice is aligned through Glastonbury Tor with the heart of the Milky Way.
The Winter Solstice of 2010 has a lunar eclipse. The eclipse would have told the visual observer when the Sun, Earth, Moon alignment was exact. Today, for a few decades, the alignment of the solistital sun and the two crossroads is focusing us upon a major turning point in the precessional cycle. It is hard to imagine a greater alignment of celestial events.
