Megalithic Astronomy

Transcription

1 Megalithic Astronomy with emphasis on Great Britain Frank Verbunt Astronomical Institute, University of Utrecht, Netherlands April 26, 2011 Frank Verbunt (Astronomical Institute Utrecht) Megalithic Astronomy April 26, / 29

2 Outline 1 Introduction the megalithic monuments archaeoastronomy 2 Sightlines and directions 3 Examples Stonehenge results by Thom on multi-site analysis 4 New research by Ruggles the Hebrides the Grampians Cork-Kerry 5 The need for restraint and critical attitude 6 Conclusions Frank Verbunt (Astronomical Institute Utrecht) Megalithic Astronomy April 26, / 29

3 the megalithic civilisation chronology Mykenae & Stonehenge (calibrated) C-14 dates: Stonehenge much older than Greek Bronze age! farming enhanced in Middle East: use crop to feed cattle, use cattle dung to grow crop arrives in southern Europe ± 5500 B.C. arrives in Great Britain ± 4100 B.C. (agricultural surplus? leads to?) large-stone monuments Frank Verbunt (Astronomical Institute Utrecht) Megalithic Astronomy April 26, / 29

4 archaeoastronomy 1740 Stukeley: main axis Stonehenge points to north-most sunrise 1905 Lockyer dates Stonehenge on this direction 1966 Hawkins: Stonehenge decoded as astronomical observatory 1966 Hoyle: Stonehenge as eclipse predictor 1967 Thom writes survey Megalithic sites in Britain criticism 1966 Atkinson: Moonshine on Stonehenge: take account of archaeological knowledge 1981 Heggie Megalihic Science: do statistics correctly 1999 Ruggles Astronomy in Prehistoric Britain and Ireland: new, correct studies Frank Verbunt (Astronomical Institute Utrecht) Megalithic Astronomy April 26, / 29

5 Defining sightlines Sightlines 1 line connecting 2 menhirs 2 symmetry axis of ring; or line from center ring to stone in ring 3 line from monument (ring, grave, menhir group) to distant stone 4 grave with corridor 5 lines between monuments (rings, graves, menhir groups) 6 line from monument (ring, grave, menhir group) to natural sight (island, mountain) 7 other (very rare) also take into account: height of horizon and atmospheric refraction accuracy of sightline (8 at Stonehenge, 1 for faraway natural foresight) is the sightline visible? (intermediate hills or forest) how many lines can one define? have stones been moved? Frank Verbunt (Astronomical Institute Utrecht) Megalithic Astronomy April 26, / 29

6 Interesting directions The Sun most northern sunset or sunrise (midsummer) sunset or sunrise at equinox most southern sunset or sunrise (midwinter) The Moon The orbit of the Moon makes an angle of 5 with the ecliptic : the Moon has extreme location of rising or setting at locations δ(moon) = δ(sun)+5 δ(moon) = δ(sun) = 6 special directions for Sun. Add 2 6 = 12 for Moon to get total of 18 directions. Note: all these lines are defined by declination δ of Sun or Moon, which convert to different azimut in different locations convert measured azimuth at horizon to declination before comparing locations. Frank Verbunt (Astronomical Institute Utrecht) Megalithic Astronomy April 26, / 29

7 Example 1: Stonehenge (Heggie p.148) Stonehenge I: ±3000 B.C. The first version of Stonehenge (Stonehenge I, ±3000 B.C.) consisted of a ring of 57 holes, and a bank and ditch just outside it. Near an opening in the latter, two stones were standing inside the bank, and four some distance out. Together with the center of the circle these define the main axis of Stonehenge. Next to the four outside stones, one or possibly two larger stones had also been erected; one of these is the Heel Stone. Frank Verbunt (Astronomical Institute Utrecht) Megalithic Astronomy April 26, / 29

8 Example 1: Stonehenge (Heggie p.148) Hawkins 1966: Stonehenge I assumed date: 1500 BC (correct: 3000 BC; not important for Sun or Moon) 18 interesting directions: set/rise Sun/Moon; accuracy ±2 probability 1 trial is: 18 4/360 = directions defined by stones of which 24 hits: probability 24 hits in 50 trials with p = 0.2 (Poisson): Atkinson 1966 correct probability 24 hits is but should be for 24 lines, and is there are in fact 111 lines and probability of 24 hits is 0.37 not significant Poisson statistics done simply 111 lines, probability 0.2 expected number of hits is 22 ± 22 (1 σ) 24 hits is within 1 σ of expectation Frank Verbunt (Astronomical Institute Utrecht) Megalithic Astronomy April 26, / 29

9 Example 2: Stonehenge II (Heggie p.198) Stonehenge II: ±2500 B.C. A later version of Stonehenge (Stonehenge II, ±2500 B.C.) added four large stones the Station stones to the ring, and according to Atkinson, but cast into doubt by later research shifted the main axis somewhat, by replacing the two stones just inside the bank. If the central ring and horseshoe made of large stones were put into location at the same time, they would have blocked the view between the station stones. Frank Verbunt (Astronomical Institute Utrecht) Megalithic Astronomy April 26, / 29

10 Example 2: Stonehenge II (Heggie p.198; updated) Example: p.198 Stonehenge Station Stones rectangle sides, diagonal: 4 directions 1 parallel main axis: 3 new 2 point to interesting directions 6 interesting directions p = 6 3.6/180 = trials P(1)=0.25, P(2)= intentional, 1 accidental ( ) n P B (k, n, p) = p k (1 p) n k k Frank Verbunt (Astronomical Institute Utrecht) Megalithic Astronomy April 26, / 29

11 Example 3: Thom 1954 on many sites (Heggie p.152) declination of Sun δ = ɛ, ɛ at solstitia between 3000 B.C. and 1500 B.C. ɛ ranged from to allow for 15 radius Sun: range becomes to which is 43 sightline selection take all lines in range 23 < δ < 25, i.e. range of lines found, all in accepted range for Sun probabiblity hit with one trial: 43/ probability 7 hits in 7 trials is < : direction to solstitia are significant separated by 30 upper and lower limb of Sun were used Frank Verbunt (Astronomical Institute Utrecht) Megalithic Astronomy April 26, / 29

12 Example 4: Thom 1967 on many sites (Ruggles p.56) Survey of British Islands select lines in lunar directions, determine difference δ to nearest lunar direction for each line, plot Gaussian with unit surface, center on δ, and width of uncertainty in δ add all Gaussians: left graph in Figure. top: all measurements, lower: only accurate measurements separation 2 peaks lunar diameter: rim of Moon observed Critique Ruggles (1999) remove lines from dubious (non-megalithic) sites: result is middle plot re-measure lines: right hand-side plot conclusion: no evidence for double peak Frank Verbunt (Astronomical Institute Utrecht) Megalithic Astronomy April 26, / 29

13 Example 5: Thom 1955 (Heggie p.159) solstitia select lines from center circle to outliers: 17 lines on 9 sites select alignments: 21 lines on 14 sites result: significant excess to solstitia and equinoxes Critique Heggie: excess due to solstitia only not all acceptable lines were counted treatment of 2 directions in one alignment is not clear stars assumed date 2100 B.C. 22 hits, probability < Critique Heggie: many lines of low quality of 9 high-quality lines 6 are on dubious sites; nonetheless 5 pointed to Rigel number of lines critical in determining probability Frank Verbunt (Astronomical Institute Utrecht) Megalithic Astronomy April 26, / 29

14 New research by Ruggles Areas of new research How to do it right To avoid biases in pre-selecting lines before statistics is done, (which is much evident in earlier work by Thom and Hawkins), Ruggles and colleagues decided to make a large, fully unbiased survey of megalithic monuments in the Hebrides. Frank Verbunt (Astronomical Institute Utrecht) Megalithic Astronomy April 26, / 29

15 New research by Ruggles The Hebrides (Ruggles p.91) Select lines in order of quality 1 row of three or more standing stones 2 row of three or more standing/lying stones 3 two stones with third stone lying 4 two stones, one lying 5 flat side of standing slab 6 flat face of lying slab On each site, keep only lines of the highest quality class. Results This leads to 296 directions with allowed ranges. The distribution of the directions is shown in the Figure (histogram) and compared with expectation according to chance (smooth line, found by arbitrarily selecting directions from the allowed ranges). Frank Verbunt (Astronomical Institute Utrecht) Megalithic Astronomy April 26, / 29

16 Results by Ruggles et al. for the Hebrides There are significant excesses near the lunar directions at rise in the southern maximum ( ɛ 5 = 28 ), and some beyond. In the northern direction the significant excess is at too high declinations. Frank Verbunt (Astronomical Institute Utrecht) Megalithic Astronomy April 26, / 29

17 Results by Ruggles et al. for the Hebrides The Hebrides If we plot the directions in terms of azimuth, we see that most directions are to the northern and southern quadrants. Left, all lines are shown; right, the ones from the Outer Hebrides (the two islands top left in the area indicated on p.15) have been removed. This looks more convincing, but at the cost of some arbitrariness in selection... Conclusions Maybe the sites were aligned on the most southern rise/setting of the Moon. Maybe they were aligned roughly in a southern/northern direction. Frank Verbunt (Astronomical Institute Utrecht) Megalithic Astronomy April 26, / 29

18 New research in the Grampians (Ruggles p.91) Ruggles & Burl (1985) measured directions in stone circles, in which one of the stones in the circle lies flat, with two standing stones next to it. The first figure shows the directions a) from the circle center across the flat stone b) perpendicular to the long axis of the flat stone; both measured in the Grampians, and showing a preference for declinations near δ = 30. For comparison similar directions measured in Cork-Kerry in Ireland are shown in c); these have no preferred direction. The second figure shows directions a) from the same circles to conspicuous hilltops b) from the circle center across a cupmark on the flat stone; c) hilltops in the Irish comparison sample. 30 is further south than the Sun comes; which may mean that the directions are to the most southern Moon. It is remarkable that the people in the Grampians and in Cork-Kerry made very similar stones, where those in the Grampians were aimed for the Moon, and those in Ireland randomly directed. Frank Verbunt (Astronomical Institute Utrecht) Megalithic Astronomy April 26, / 29

19 the Grampians: direction across flat stone Frank Verbunt (Astronomical Institute Utrecht) Megalithic Astronomy April 26, / 29

20 the Grampians: from circle to faraway hilltop Frank Verbunt (Astronomical Institute Utrecht) Megalithic Astronomy April 26, / 29

21 New research in Cork-Kerry (Ruggles p.102) Ruggles also investigated the rows of stones in Cork-Kerry. He finds a preference for directions δ 30 ; since the Sun at the time was at δ 24, this suggests that the Moon, rather than the Sun was the target of the alignment ( δ (Moon)= δ (Sun)+5 at the extremes). In some cases the row of stones points in one direction; in some case two directions are feasible. The latter cases are included in the lowest graph. Rows of stones may also point to hilltops in the distance. The directions found in this way, again for Cork-Kerry, are shown in the Figure. For comparison, the lowest graph shows similar directions for rows and hills in western Scotland. It appears to me that the southern directions are compatible with being purely solar. Frank Verbunt (Astronomical Institute Utrecht) Megalithic Astronomy April 26, / 29

22 Cork-Kerry: rows of stones Frank Verbunt (Astronomical Institute Utrecht) Megalithic Astronomy April 26, / 29

23 Cork-Kerry: rows pointing to distant hilltop Frank Verbunt (Astronomical Institute Utrecht) Megalithic Astronomy April 26, / 29

24 The need for restraint and critical attitude Be critical Many publications still appear that claim alignments that are not substantiated by a valid statistical analysis. Such alignments are not believable. In particular studies of sites outside Europe appear to be still in the pre-heggie era of archaeoastronomy, and prone to over-enthusiasm. Examples The reader may test her/his critical acumen on The Orion mystery by Dauval & Gilbert (1994) claims that the pyramids of Gizeh are an image of the Orion constelllation Sterne und Weltraum 421, 34 (2003) claims that the Nebra disk is a map of the sky National Geographic May 2004 claims an alignation of Maya buildings in Guatemala with η Draconis Frank Verbunt (Astronomical Institute Utrecht) Megalithic Astronomy April 26, / 29

25 Assorted garbage National Geographic May 2004 alignment on η Draconis... Frank Verbunt (Astronomical Institute Utrecht) Sterne und Weltraum December 2003 sky map with Pleiades... Megalithic Astronomy April 26, / 29

26 Conclusions on Megalithic Astronomy Megalithic structures occasionally show preferred directions, which may be interpreted as astronomical. Early research (before 1980) suffered from flaws in the statistical treatment, but was important in developing an interest in the field. Since the 1980s better defined research has been done, and from this it follows there are significant directions towards most northern and southern sunsets/sunrises (the solstitia) there may be significant directions towards the most northern and southern moonrise/moonset; alternatively they could be towards the sun, if the builders did not strife for large accuracy, but merely for a rough indication of direction. there are no significant directions towards stars Significant here means: it can be shown from a statistical analysis that the frequent occurrence of the direction is unlikely to be due to chance. Frank Verbunt (Astronomical Institute Utrecht) Megalithic Astronomy April 26, / 29

27 Conclusions on Megalithic Astronomy It should be noted that astronomical explanations are not the only possibility. An example of very different interpretations is the location of megalithic sites on the north side of the island Mull. The sites are often on locations on the lines which separate the areas where one can see the mountain Ben More from those where one cannot (Ruggles, p.115). It should be noted that statistical analysis has its limits. That we cannot prove something statistically doesn t prove that it is not the case; in particular, single sites cannot be analysed in a statistical way. For example, if three henges are roughly in positions looking like the belt of Orion, this may or may not be what the builder intended we cannot know. Frank Verbunt (Astronomical Institute Utrecht) Megalithic Astronomy April 26, / 29

28 Literature Advised reading Douglas Heggie, Megalithic Science, Ancient mathematics and Astronomy in Northwest Europe, 1981, Thames & Hudson. History of topic, and items 1-3 above. An absolute must for any student of megalithic astronomy. and: Clive Ruggles, Astronomy in Prehistoric Britain and Ireland, 1999, Yale University press. Three parts: as Heggie, less concise new research (item 4 of Outline) synthesis with archeology Well illustrated; rather verbose in places (esp. 3rd part). Frank Verbunt (Astronomical Institute Utrecht) Megalithic Astronomy April 26, / 29

29 Exam problem 1: precession and Stonehenge Sunrise at Stonehenge Stonehenge is at N. 1 Compute the most northern azimuth of sunrise at Stonehenge in 2000 AD 2 with precession at constant obliquity ɛ, compute the most northern azimuth of sunrise at Stonehenge in the year 3000 B.C. 3 with full precession compute the most northern azimuth of sunrise at Stonehenge in the year 3000 B.C. Stonehenge ±3000 B.C. 4 is it possible to date Stonehenge from its main axis? Explain your reasoning. Frank Verbunt (Astronomical Institute Utrecht) Megalithic Astronomy April 26, / 29