Callanish, Cronus and a Mysterious ‘Stranger’
Two essays from Plutarch’s Moralia describe voyages made by the North Britons to an Atlantic island, which Plutarch equates with the Ogygia of Homer. Plutarch gives two related references to the voyages, the first in ‘Obsolescence of Oracles’ is brief; and the second in ‘The Face in the Moon’ offers more detailed geography. Conventional scholarship would view these references as little more than an attempt by Plutarch to equate some Celtic myths with those of Greece. However, an examination of the underlying astronomy and the aligned Neolithic monuments of Britain would suggest that there is much more to it than that; and that they offer us clues to the true antiquity of the Druids and their calendar.
In Plutarch’s moral essay, Obsolescence of Oracles (De Defectu Oraculorum) one of the speakers is a certain Demetrius, who in an aside remarks upon the eternal imprisonment of Cronus.  He relates that Cronus was confined in a cave on an island close to Britain, guarded as he slept by the ancient Briareüs and various other daimones (demi-gods). In the recognised Greek mythology Cronus was ruler of the Titans and the father of Zeus; the Romans would equate him with their own god Saturn.
In The Face on the Moon (De Facie Quae in Orbe Lunae Apparet) Plutarch goes further and gives us a better opportunity to identify the geography; moreover, he identifies one of the islands with the Ogygia of Homer’s Odyssey.  This island, supposedly lay some five-days sailing west of Britain in the direction of summer sunset; three other islands lay in-between, equidistant from the mainland and from each other. On this distant isle was an oracle where the faithful could receive the prophecies of Cronus in his dreams. He also refers to this part of the North Atlantic as the Cronian Sea and says that sometimes it could become ‘congealed’ (frozen?) but the geography is rather blurred and we cannot be sure quite what is intended, or indeed on which island we should seek the sleeping Cronus. It may be that Plutarch himself was confused by the geography in his sources and merely passes it on to his own readers.*
The Loeb translations of these essays are now freely available in the open-sources (linked above) and so need not be quoted here in full. However, it is important to note that the source text for The Face in the Moon is incomplete; the beginning of the essay is missing.
Beyond the Cronian Sea was the ‘great mainland’; evidently a continent that lay a further 5,000 stades beyond Ogygia, across the Cronian Sea. On a gulf of its coast, resided a colony of Greeks who had migrated there from Britain via Ogygia. They believed they were the descendants of Greeks who, in ancient times, came west in the train of Heracles and over a long period of years had merged with the people of Cronus. Thus, they still revered Cronus, second only to Heracles.
Plutarch reveals that regular expeditions to this distant island continued right up to his own day; the voyages commenced from an island off the coast of Britain, which there are good reasons to suggest was the Isle of Lewis. Every thirty years when the Star of Cronus (the planet Saturn) returned to the constellation of Taurus, the crossing would begin. The faithful would set-off in a number of rowing boats, putting-in at outlying islands on the way (also occupied by Greeks) before the long haul to Ogygia began.
Demetrius is introduced as a grammarian who discussed philosophy with Plutarch and Cleombrotus of Sparta during a meeting at Delphi, on his way home to Tarsus; the date is identified as 83-84 AD by reference to the Olympiad.  Demetrius says that he was instructed by the emperor to survey the islands beyond Britain, but we cannot be sure which emperor sent him: Vespasian or Titus. He mentions many isolated islands and he had himself visited one of the nearer isles, inhabited only by a few holy men from whom he had learned the story of Cronus. The particulars are repeated in The Face in the Moon, by the Carthaginian, Sextius Sulla; he argued that many of the Greek myths were wrong and adds the important detail that the story came directly from the native Britons – he is not citing an earlier Mediterranean author. This dialogue took place later than AD 84 and so Plutarch could already have been influenced by Demetrius. A solar eclipse, probably that of 5 January AD 75, is mentioned as ‘recent’, but the best opinion of the translators will only put the date of the dialogue as sometime much later than this.  Plutarch died in AD 120.
Confirmation that the starting point for the Atlantic voyages was the west coast of Scotland comes from the astronomy. It is perhaps best here to simply quote the words of Sulla. Firstly, he cites Homer:
An isle, Ogygia, lies far out at sea,
a run of five days off from Britain as you sail westward; and three other islands equally distant from it and from one another lie out from it in the general direction of the summer sunset. In one of these, according to the tale told by the natives, Cronus is confined by Zeus, and the antique Briareüs… 
Later he comments:
The great mainland, by which the great ocean is encircled, while not so far from the other islands, is about five thousand stades from Ogygia, the voyage being made by oar… 
The statement that the direction of the voyages was towards the summer sunset tells us that the location of Ogygia lay to the northwest. Iceland lies some 670 miles (1085 km) northwest from Lewis, but whether this could be sailed or rowed in just five days seems improbable; and the distance of a further 5,000 stades to the distant mainland beyond the Cronian Sea, suggests a voyage of about 575 miles (925 km).
We are also offered a latitude for the final destination: a gulf lying upon a similar parallel to the Caspian Sea. This extra detail would equate it with Newfoundland or perhaps Nova Scotia rather than Greenland. It is here that the muddled geography defeats us. It is sometimes not clear, when Plutarch goes on to discuss the ‘mainland’, whether he intends the mainland of Britain or whether he refers to the distant ‘great mainland’ beyond the ocean. Moreover, we cannot distinguish whether the cave of Cronus supposedly lay on one of the islands on the way to Ogygia (Iceland?) or on one of the islands between Ogygia and the great mainland beyond. Was the colony of ‘Greeks’ situated on the ‘great mainland’ or was it on the mainland of Britain? This lack of clarity has led to a variety of speculations over the years.
Some have suggested that the narrative describes a journey via Iceland, Greenland and Labrador to Newfoundland – analogous to the later Viking sagas. More conservative classical scholars might prefer that it describes a shorter journey via the Inner Hebrides from mainland Britain to the Outer Hebrides. Others more conservative still cannot conceive that Homer’s Ogygia lay beyond the fictional Mediterranean theatre. However, although the Isle of Lewis may sometimes seem remote to outsiders there is certainly no need to wait thirty years to go there; and the voyage was surely not so daunting even in ancient times! None of the explanations quite fits Plutarch’s geography.
The narrative then moves on to discuss the ‘stranger’ who returned from the distant island. He is not named and it may be that he was introduced in the lost beginning of the essay.
Here then the stranger was conveyed, as he said, and while he served the god became at his leisure acquainted with astronomy, in which he made as much progress as one can by practising geometry, and with the rest of philosophy by dealing with so much of it as is possible for the natural philosopher. Since he had a strange desire and longing to observe the Great Island (for so, it seems, they call our part of the world), when the thirty years had elapsed, the relief-party having arrived from home, he saluted his friends and sailed away… 
We cannot be sure whether the stranger passed all of his religious servitude in Ogygia, or on the distant mainland, or on some intermediate isle where Cronus was believed to sleep; such is the uncertainty. All that really matters is that after thirty years he returned. We may wonder what was so ‘strange’ about him? Perhaps he was a native, born on the distant isle? Plutarch tells us that eventually he made his way to Carthage where Cronus was still a much-revered god.  There he met with Sulla, who later narrates the details to Plutarch.
The astronomical circumstances as they are given allow us to retro calculate the date of the events. We already know from the narrative that Demetrius was in Britain before AD 84, which was the era when Agricola took Roman forces into Caledonia. There is little reason to doubt that Demetrius of Tarsus was a real historical person. Any visitor to the Yorkshire Museum may see two votive offerings in Greek, left by a visitor named Demetrius at the site of the Roman legionary fort (RIB662 & RIB663). The first fortress at Eboracum is believed to have been built in AD 71as the base for operations further north. We can be sure that Demetrius made his explorations to the west coast earlier than AD 84, probably in AD 82 protected by Agricola’s army. 
The thirty-year return of Saturn to the constellation Taurus gives three possible dates for the ‘stranger’: AD 57, AD 87 and AD 117. No precise observational astronomy is needed; both the local observers and those in the distant colony would know that when Saturn returned to the proximity of the Hyades or the Pleiades then the relieving voyage should begin in the following summer. It is most likely that he made his pilgrimage west in AD 57 and returned in AD 87, allowing time for his later experiences in Carthage to have taken place during Plutarch’s lifetime. We cannot rule out that his voyage occurred in an earlier thirty-year cycle but we can rule out AD 117. Nothing in the narratives requires that Sulla and Demetrius ever met each-other. However, Sulla is directly quoting his conversations with the ‘stranger’ when he says that some Greek myths were incorrect; and that ‘stranger’ had learned these specifics directly from the chamberlains at the oracle of Cronus.
Below: three views thirty years apart showing Saturn in Taurus as it sets in the west, with the moon near conjunction. Taurus is a sprawling constellation and so its centre was probably taken as either the Hyades or the Pleiades. [retro calculations via Skymap Pro]
3 January 57 AD
3 January 87 AD
3 January 117 AD
Pliny, in his Natural History also offers much that is revealing. In discussing the calendar of the Druids, he too mentions the thirty year ‘ages’ – but he does not make the link with Saturn. 
…above all on the sixth day of the moon (it is the moon that marks out for them the beginnings of months and years and cycles of thirty years) …
At this festival, a bull sacrifice was performed, showing us that this long cycle had both a religious and a practical significance; and we may presume there were other seasonal festivals. Pliny’s geography of the Western Isles and the polar seasons is also informative. After detailing the islands around Britain, he goes on to describe the distant northern island of Thule:
The most remote of all those recorded is Thule, in which…there are no nights at midsummer… and on the other hand no days at midwinter.
..Some writers speak of other islands as well, the Scandiae, Dumna, Bergos, and Berrice the largest of them all, from which the crossing to Thule starts, called by some the Cronian Sea. 
The largest island: Berrice, is plainly the Isle of Lewis – for he has previously counted all the Orkneys, Shetlands and Hebrides. The voyage across the Cronian Sea to Thule must surely recall the same sea-crossing as does Plutarch, only the name of the destination changes.
Plutarch prefers the name Ogygia. The name Thule has come via earlier geographers and ultimately from the fourth century BC voyage of Pytheas, of which Pliny observes:
Pytheas of Marseilles writes that this [the polar seasons] occurs in the island of Thule, 6 days north from Britain. 
This comment would fit well with the five-day voyage to Ogygia as given by Plutarch, except that a heading due north from Lewis would take them to the Faeroe Islands not to Iceland; perhaps these were one of the intermediate islands as described. Again, the vague geography defeats conclusive identification; today we know the true geography, but it is understandable that the ancient geographers assumed different names for the same island to be different locations. Strabo, who treated the voyage of Pytheas as a fiction, gives an account of the climate and inhabitants of Thule that would fit better as a description of the Outer Hebrides. 
In the Loeb translation (p 181-183) Cherniss and Helmbold comment that Plutarch’s account of the ‘great mainland’ beyond the ocean was likely inspired by Plato’s Atlantis; and his remark about the congealed sea to be a reference to the debris of the sunken island. Such a comment is typical of conservative classical scholarship, which would treat Greek and Roman writers as the only legitimate source of ancient wisdom; all ancient texts are to be viewed merely as classical authors citing each other and ultimately as fictional recreations of myths. To discuss Plato’s narrative as a degraded memory of something real would be to enter the realm of the lunatic.
In Timaeus, Plato describes the Atlantic voyages as taking place before the sinking of the lost island (and thereby implies that the knowledge was ancient). Via the island it was formerly possible to sail to the continent on the far side of the ocean before the way was blocked and forgotten.  if we accept the internal content of the underlying sources then we have one story of a continent beyond the ocean that ultimately came from the temple of Neit in Egypt; the other came via native British tradition; both remembering similar historical geography. However, we wander off-subject to pursue these topics further.
Why should modern historians find the notion of ancient transatlantic voyages so implausible? At this same era and for thousands of years the Polynesian explorers were rowing across the vast Pacific, navigating by the stars. Why then deny this same capability to European mariners? However, such exploration requires acceptance of a higher standard of navigational astronomy than is usually ascribed to northern ‘barbarians’ at this era. This is another text-book preconception of classical scholarship that has to be challenged. Naked-eye astronomy is not difficult; it merely requires diligence and a reliable calendar against which to record and predict the movements of the celestial bodies. We should never lose sight that ancient astronomy was actually just precise astrology, to interpret the signs of the ‘visible gods’ in the sky.
We should also consider that the inhabitants of the Western Isles and Ireland may observe the echelons of migratory geese and swans flying off to the northwest and returning again each year. They could reasonably assume that land existed in that direction, just as Polynesian navigators would look for these indicators to find islands in the vast Pacific. Long before any Greek or Carthaginian navigators reached Britain, we may presume that local fishermen were venturing out into the Atlantic and using the sky above for navigation.
Callanish and the Stone Circles
On the Atlantic coast of Lewis, on the inlet of Loch Roag lies the Callanish stone circle. Although the oldest stones date from as early as 2900 BC, the unique cross-shaped monument that we see today is a relatively late construction, dating from after 2000 BC. The cruciform arrangement disguises the fact that at its heart is a stone circle with a central monolith and a (later) chambered cairn built within. Beaker pottery has also been found at the site. The monument fell into disuse around 800 BC and in the damp climate it began to be enveloped in a layer of peat.
The original stone circle presumably had the same calendrical and religious purpose as all the other circles. So, what was its original purpose? The latitude of Callanish is such that the lunar standstills, when at their most southerly extreme every 18.6 years, rise and set behind the hills on the southern horizon. However long rows of stones are not essential to mark an alignment and so it is more likely that these possessed some processional significance for religious ceremonies. The inlet of Loch Roag is a prime candidate as the port of embarkation for ancient voyages into the western ocean and so Callanish may have retained a ritual significance as a place of pilgrimage for the later Druids long after its original astronomical usage was forgotten.
Left: the Layout of the circle and avenue at Callanish
(click on the diagram for access to Google Maps).
Again, we would go off-subject to pursue a detailed discussion of the Callanish alignments here. The astronomical alignments have been well studied, one of the earliest being Captain Boyle Somerville in a 1912 article in Nature.  He thought he could see some navigational purpose to the alignments, suggesting orientations to Capella and other prominent stars – but his work predated any accurate means of archaeological dating. Later investigators such as Hawkins and Thom proposed the lunar horizon alignments back in the 1960s,
A recent study by Dr Gail Higginbottom would claim to have statistically proved the alignments for the first time.  The paper provoked popular responses that Callanish and other similar monuments must have been laid-out by ancient astronomers. This would be rather like saying that the people playing football in the stadium must be footballers. Why should it be required for an academic to state the obvious in an appropriate journal in order for it to be accepted as fact? The ancient builders did not have the luxury of precise modern computations and surveying equipment. An approximate alignment to a seasonal marker was good enough for purpose and the best way to attain accuracy was to align to the celestial body as it rose or set at the furthest point on the horizon. They simply made use of whatever horizon feature was available at each site.
Rather we should perhaps be asking, what compulsion drove the Neolithic people to build these outdoor stone alignments all over Britain and beyond. Initially there must have been a practical imperative to understand the seasons, but later this lapsed into religious ritual.
Some commentators on Callanish, going back to John Toland in the eighteenth century, have thought that the ‘temple of the spheres’ in the land of the Hyperboreans beyond the Celts, as described by Hecataeus (quoted by Diodorus Siculus) is more likely to have been Callanish than Stonehenge; it was more accessible by sea and would better comply with the statement of Hecataeus that at this temple the moon came very close to the earth.  This could describe the coincidence that when at its most southerly extreme the moon just skirts the horizon at this latitude; however, against this it must be said that the same seasonal lunar indicators would still be needed at every stone circle, regardless of latitude. **
If one may digress at this point to consider Stonehenge, as being illustrative of the function of other circles, then we have another aligned ‘temple’, dating from the same era; like Callanish it also passed through later stages of renewal. If we look beyond the giant stones that dominate the site then we also have the ring of 56 chalk-filled holes, known as Aubrey holes after their seventeenth-century discoverer. These are thought to date from the first phase of the monument around 3000 BC and may once have held wooden posts, or perhaps the bluestones before the monument was reorganised. Around the main sarsen ring are the Y and Z holes. However, these have been dated to the later phase of the monument when the giant sarsen stones were erected.
Since there are 29 of the Z-holes and 30 Y-holes these have long been assumed to represent the days of lunar months. Therefore, it is reasonable to suggest that the 56 Aubrey holes also served a practical calendrical purpose for the original builders.
Theories that Stonehenge was an observatory to predict eclipses were proposed by Gerald Hawkins in 1963.  This was followed by the more general theories of Alexander Thom of solar and lunar alignments at many other stone circles. 19] Following Hawkins and Thom, in 1977 the renowned cosmologist Sir Fred Hoyle took up the subject of the Aubrey holes. His scheme built upon Hawkins theory that Stonehenge, at least its earliest phase, was as an elaborate observatory to predict eclipses. However, he would dismiss any connection between the megalith builders and the later Druids.  His introductory words on page one are important to note for the historiography of this subject:
The Druids had nothing to do with the construction of Stonehenge, which was completed a thousand years before the Celtic peoples invaded the British isles.
Alas, again, this kind of statement is a delusion of modern scholarship: the assumption that ancient wisdom is only as old as its earliest mention in a classical source. It is rather like saying that the medieval cathedrals could have had nothing to do with Jesus because they were built more than a thousand years later. Of course, ancient knowledge and religion could have survived to form part of druid teaching. In saying this, Hoyle, who was performing a cross-disciplinary study outside his expertise, was merely citing the accepted textbook wisdom of the day. This preconception persisted well into the twenty-first century until DNA science showed that the so-called Celtic invasion of the Iron Age never happened. Once this preconception is removed then we may readily accept the statements by contemporary historians that Druidism originated in Britain.  We may also see that Anglesey was the ‘Jerusalem’ of their cult, which might explain why the Romans went to such lengths to eliminate it as a centre of resistance.  Druid astronomy and wisdom was ancient; Celts in Gaul and elsewhere were converted to it, rather than the other way about.
Hoyle would build upon the 18.6-year period of the lunar standstill and conclude that the 56 Aubrey holes were a device for predicting the eclipse cycle, based on three such cycles or 55.8 years. Because of the prejudice that there was no link with the Druids, he did not consider either Saturn or the thirty-year druid cycle. The same may be said of the other modern investigators who were similarly constrained to consider the megalith builders as stone-age ‘pre-Celts’.
In fact, as I showed in Under Ancient Skies in 2005 and in two later articles the Druids maintained an accurate lunisolar calendar based on an 11-year cycle. The number 56 provides the link between Saturn and the Moon.  There is an equivalence between the Moon and the synodic period of Saturn (the period between each opposition) that would be noticed by any society that sought to schedule its motion against an accurate lunar calendar:  
717 lunar months = 717 x 29.530598 = 21173.43days = 57.97 solar years
56 synodic periods = 56 x 378.09292 = 21173.2 days (approx. 58 years)
The discrepancy is only about five hours and ancient people may well have considered the correspondence to be exact. It would seem much more likely that this equivalence between Saturn and the Moon was the significance of the 56 Aubrey holes rather than for predicting eclipses. There is only one chance in fifty-six that the choice of this number was a random coincidence.
A similar solar correspondence occurs after 59 years:
59 solar years = 59 x 365.2422 = 21549.29 days
57 synodic periods = 57 x 378.09292 = 21551.29 days
Therefore, after 59 years the configurations of Saturn in the sky are repeated exactly two days later in the solar year. Either or both of these correspondences could then be used to check the accuracy of a lunisolar calendar and allow corrections to be made after 60 years, analogous to the Gregorian century rule.
We should also note the astronomy of the 30-year cycle and Saturn.
29 synodic periods = 29 x 378.09292 = 10964.694 days = 30.02 solar years
The discrepancy is only about a week or one quarter-moon; and 30 plus 29 also gives the 59-year checkpoint. Too many coincidences here.
Therefore, we may see that the rings of 29 and 30 post holes at Stonehenge need not represent the months at all, but rather they may be related to Saturn rituals. The occultations of Saturn by the Moon may also have been significant astrological events for astrology, divination and ceremony. Once we remove the artificial constraint that druid science was ‘Celtic’ and accept that their astronomy was older, then it becomes apparent what they were doing. They meshed their lunisolar calendar with Saturn in the same way that the Mayans and the Babylonians meshed theirs with cycles of Venus; and they used it to validate the day count and maintain its accuracy.
Plutarch’s essays supply the missing link; they show the relationship between the thirty-year druid cycles, the ancient cult of Cronus, and his visible star: the planet Saturn.
There are no references in classical sources to suggest that the Britons took any interest in eclipses, whereas we do find clear references to Cronus and the thirty-year druid cycle. The complex eclipse theories proposed by the modern specialists, to explain the function of the stone circles, have been another diversion that has led us in the wrong direction.
Neither is Plutarch the only classical author to offer parallels between Greek and ‘Celtic’ mythology. Also neglected have been the various fragments of lost knowledge that would equate Celtic gods and myths with those of the Mediterranean. In Caesar’s commentaries, probably summarising Posidonius and earlier historians, we are told that the doctrine of the Druids was imported into Gaul from Britain where it was found already existing. The Celts of Gaul, he says, recognised all the same gods as the Romans, but curiously he fails to mention Cronus! It may be that the cult of Cronus/Saturn was best remembered only in Britain and the North.  The ‘stranger’ told Sulla that one should especially honour the moon (the goddess Cora) demonstrating once again the importance of the Saturn-Moon relationship.
Hecataeus and Herodotus, writing hundreds of years earlier, said that the northern Hyperboreans had maintained a friendship with Athenian and Delian Greeks since the most ancient times, exchanging both gifts and visitors with each other.  Raffael Joorde has recently suggested that the ‘travel guide’ by Hecataeus of Abdera drew on the lost book ‘On the Ocean’ by Pytheas.  ***
Also significant must be the conviction of Plutarch that some Greeks had migrated to the west of Scotland in the era of Heracles. Perhaps DNA science can one-day offer some insight on this. In the Irish Book of Invasions (Lebor Gabála Érenn) we are told of successive waves of invaders from ‘Greece’ via Spain: Nemedians, Firbolgs and Danaans. After the Firbolgs were defeated by the Danaans some of these earlier ‘Greeks’ were driven from Ireland to take refuge in the Scottish islands, among a shadowy indigenous people called Fomorians. Does this not echo the same story that Plutarch gives us? And where should Heracles fit into all this?
So far as we know, there is no archaeological evidence of Neolithic occupation on Iceland or the Faeroes. The lost cave and oracle of Cronus remains for archaeologists one day to find!
The coincidence that Saturn made one of its 30-year returns to the constellation Taurus, around the same date that the explorer Demetrius was in Britain confers an authenticity on the story of the stranger’s voyage as also contemporary with Agricola. There would be only one chance in thirty that this was coincidence. The ‘stranger’ therefore most likely made his pilgrimage to Ogygia and to the oracle of Cronus thirty years earlier in AD 57, returning in AD 87. This would have given ample time for his subsequent visit to Carthage to be recorded and conveyed to Plutarch during his later life, but probably after he had already been influenced by Demetrius.
Plutarch’s use of the name Ogygia, rather than Thule, for the distant island tells us that his inspiration has come via a Greek source, probably Demetrius, rather than from older sources such as Pytheas and those geographers who cited his book. Plutarch, or his source has therefore equated the two locations and preferred the Greek name. The Ogygia of Plutarch and the Thule of Strabo and other geographers are the same place – Iceland. Whether this was the same place as Homer’s Ogygia is a complication we need not pursue here.
The voyage to Thule/Ogygia most likely began from various points on the British mainland and called at the inner and Outer Hebrides, where the pilgrims assembled their fleet of rowing boats before embarking on the long journey to Thule. The convoy most likely set out from the east coast of Lewis as may be inferred from Pliny. Therefore, Plutarch’s description of the journey may be an amalgam of two similar-sounding voyages; firstly, a regular religious pilgrimage via the Hebrides to Ogygia, combined with a recollection of less frequent expeditions to Greenland or America beyond. These Atlantic crossings may have been as distant in time to Plutarch as the Vinland sagas are to us.
Such knowledge as we can assemble from the various sources would suggest that the religion of Cronus was truly ancient and must go back at least to the building of Callanish, Stonehenge and the other stone circles. The ancient calendar as preserved among the Druids was concerned not only with the short-term lunisolar cycle, but also with long-term observation of Saturn – the star of Cronus – and its geocentric orbit. The coincidence of the 56-year correspondence between Saturn and the Moon is too precise to be dismissed as coincidence. Astronomy doesn’t lie! These longer cycles would give an opportunity for the calendar to be adjusted, equivalent to the century rule of the Gregorian calendar. To accomplish this, they would almost certainly have needed an era and a system of calendar dates, similar to those we find in the Mayan calendar. Unfortunately, this ancient knowledge is lost because the British Druids refused to commit their secrets to writing.
The antiquity of the religion of Cronus and thereby of the calendrical astronomy is given by the age of the aligned monuments – Callanish, Stenness, Stonehenge, Newgrange and others dating from the late fourth millennium and early third millennium BC; and also, by Plato’s Timaeus, which if taken at its word, describes an account preserved in Egypt of similar ancient voyages into the Atlantic. The temple of Neit dates from as early as the First Dynasty of Egypt (c.3100 BC). Therefore, we may posit that the myths surrounding Heracles, Cronus and the other Titans take us back to even earlier times: to the earlier Neolithic and Mesolithic of the North.
As to when the regular 30-year voyages finally ceased, this depends upon how long you believe the ancient religion and the culture that supported it could have endured in the Hebridean Isles of Scotland. It may have persisted until the Romans under Constantine raided and subdued the northwest tribes sometime around 313 AD.  In their enfeebled state the tribes were then absorbed among the Picts from the North and the Scots from Ireland and their ancient ways were forgotten. Certainly, after Saint Columba and others introduced Celtic Christianity to the Western Isles then the old religion would have been supressed. We may wonder if some vestigial memory of the Atlantic voyages were recorded by later Christian monks; and whether the ancient knowledge encouraged Viking expeditions to Iceland and Vinland beyond.
* Note 1: A further possibility here is volcanic ash accumulating on the sea from one the many Icelandic volcanoes after an unusually strong eruption; an event that could perhaps be dated by specialists.
** Note 2: On a related subject, it will be instructive to read a recent paper by Rick Doble for an example of how preconceptions about the capabilities of Greek astronomers held-back an understanding of the Antikythera mechanism by at least 50 years.
*** Note 3: The thorough analysis by Raffael Joorde of the surviving fragments of Hecataeus of Abdera concludes that the Hyperboreans on an island beyond the Celts were a description of the inhabitants of southern Britain; and that their culture survived in a recognisable form, distinct from the continental Celts, up to his own era: c.300 BC. https://independent.academia.edu/RaffaelJoorde
Plutarch, Obsolescence of Oracles, 419, 18
Plutarch, The Face on the Moon, 941-942
Plutarch, Obsolescence of Oracles, 410,1-2
See the Loeb translation by Harold Cherniss and William Helmbold, page 11 commentary on the narrative at para 931,19.
Plutarch, The Face on the Moon, 941,26
ibid, 942, B
Diodorus Siculus, V, 66, 5
Pliny, Natural History, XVI,250
Pliny, Natural History, IV, xvi, 102-104
Pliny, Natural History, II, lxxv, 187
Strabo, Geography, I, 4, 3
Plato, Timaeus, 25
Somerville, B. (1912) Astronomical Indications in the Megalithic Monument at Callanish, J. Brit. Astron. Assoc., Nov 1912, pp 83-96
Gail Higginbottom, Roger Clay, (2016) Origins of Standing Stone Astronomy in Britain: New quantitative techniques for the study of archaeoastronomy, , Volume 9, 2016, Pages 249-258, ISSN 2352-409X
Hecataeus of Abdera, as quoted in: Diodorus Siculus, Book II. 47, 1-7
Hawkins, Gerald. F. (1966) Stonehenge Decoded, Souvenir Press, London
Thom, A. (1967) Megalithic Sites in Britain, Oxford University Press, Oxford
Hoyle, F (1977) On Stonehenge, Heineman Educational, London
Caesar, The Gallic War, VI, 13
Tacitus, Agricola, 18
Dunbavin, P (2005) , Under Ancient Skies, Third Millennium Publishing, Nottingham, ISBN:0-9525092-2-5
Dunbavin, P (2018) On the Coligny Calendar and the Neolithic Calendar in Plato’s Critias, in Chronology & Catastrophism Review, 2018:2 pp 50-53
Dunbavin, P (2020) The Neolithic Calendar, in Prehistory Papers, pp 13-22, Third Millennium Publishing, Beverley, ISBN: 978-0-9525029-4-4 (a previously unpublished 2006 paper)
Caesar, The Gallic War, VI, 13
Hecataeus cited in Diodorus Siculus. II. 47, 1-7
Joorde, Raffael; Dortmund; Hecataeus of Abdera and his work “On the Hyperboreans“ (about 300 BC): The fragments with a historical commentary
Eusebius, The Life of Constantine, IV, c, 50
Tags: Plutarch, Oracles, Cronus, Saturn, Ogygia, Thule, Cronian Sea, Callanish, Aubrey Holes, stone circles
Copyright: Paul Dunbavin & Third Millennium Publishing, May 2021, V 1.2