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Sparavigna, A. (2016). A possible role of Alpha Crucis in the astronomical landscape of Silbury Hill. PHILICA.COM Article number 616.

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A possible role of Alpha Crucis in the astronomical landscape of Silbury Hill

Amelia Carolina Sparavignaunconfirmed user (Department of Applied Science and Technology, Politecnico di Torino)

Published in astro.philica.com

Abstract
The paper is proposing a possible link between the construction of the Silbury Hill, the prehistoric artificial mound near Avebury, and the observation of Alpha Crucis, the main star of the Crux constellation, which was slowly disappearing from the local sky due to the precession of the Earth’s axis. For the discussion, we used simulations of the local astronomical landscape made by means of Stellarium software.

Article body



 

A possible role of Alpha Crucis in the astronomical landscape of Silbury Hill

 

Amelia Carolina Sparavigna

Politecnico di Torino

 

The paper is proposing a possible link between the construction of the Silbury Hill, the prehistoric artificial mound near Avebury, and the observation of Alpha Crucis, the main star of the Crux constellation, which was slowly disappearing from the local sky due to the precession of the Earth’s axis. For the discussion, we used simulations of the local astronomical landscape made by means of Stellarium software.

 

 

Silbury Hill is a prehistoric artificial mound near Avebury. Being 40 meters high, [1] it is the tallest prehistoric man-made mound in Europe [2] (Figure 1). The artificial hill was constructed in several stages, probably completed in around 2400 BC [3,4]. Since technical skills and a long-term control over works and resources were necessary for its building, the existence of a theocratic elite in the southern Britain was proposed [5], able of supervising the construction of the Hill. As told in [1], in an archaeological research that lasted from 1968 to 1970,  Richard Atkinson undertook excavation at the Hill, finding material suggesting a Neolithic date of the mound. He argued that the hill was constructed in steps, indicating a date for the commencement of Silbury close to 2750 BC [6].

What was the purpose of this hill? We can find some assertions in [1]. John Barret, in [7],  notes that any ritual at Silbury Hill would have involved physically raising a few individuals far above the level of everyone else. These few individuals were then in a privileged position, possibly indicating an elite group, perhaps a priesthood, having some authority. Writer and prehistorian Michael Dames has proposed a theory of seasonal rituals, in an attempt to explain the purpose of the Hill and of its associated sites [8]. Paul Devereux observes that Silbury and its surrounding monuments appear to have been designed with a system of inter-related sightlines [1]. Jim Leary and David Field, after an overview of the site, concluded that the actual purpose of this artificial earth mound cannot be known and that the multiple and overlapping construction phases suggest that the process of building was probably the most important thing of all [9].

Let us consider the proposal of John C. Barret, that is viewing the hill as a place where a few individuals  could have been in a privileged position and link this fact to the astronomical landscape of the site at the time of its construction. Let us also consider that the rising and setting of the stars is influenced by the precession of the Earth's axis (precession of equinoxes). And then, due to this phenomenon, ancient people could see new stars appearing above the horizons and old ones disappearing below it.

In fact, as observed by Giulio Magli in [10], astronomy was a familiar presence in ancient sites, and that, probably, sanctuaries "oriented to the brilliant stars of the southern sky,” such as those of the Crux-Centaurus group existed. But the stars of this group were “slowly disappearing from the Mediterranean sky due to precession [10,11]". Of course these stars could have been important for people at northern latitudes too, such as those living near Avebury and Stonehenge.

Therefore, let us consider the Crux-Centauri, and using the software Stellarium, let us simulate the astronomical landscape seen from the Silbury Hill, at the time of its construction. In the Figures 2, 3 and 4 we can see the snapshots of the simulation for 3000 BC, 2750 BC and 2500 BC. We have from the Figure 2 that, about 3000 BC, Acrux was above the horizon (Acrux, that is Alpha Crucis, the brightest star in the constellation Crux, the Southern Cross; it is the 13th brightest star in the night sky). In the Figure 3, we can see that this star was slightly below the horizon. In the Figure 4, we can see it is below the horizon, no more visible.

As the simulation is showing, during the building of Silbury Hill, Alpha Crucis was disappearing below the horizon. May be, the Hill was built as an astronomical observatory, where privileged people could observed this star, no more visible from the ground.

In the Figure 3, where we can see the sky at the time when the building of the hill started, the star is slightly below the horizon, so we could image it were not visible. However, we have to consider an important fact, the atmospheric refraction. This refraction is the deviation of light from a straight line as it passes through the atmosphere due to the variation in air density as a function of altitude. Astronomical refraction causes stars to appear higher in the sky than they are in reality. The refraction of the light from a star is zero in the zenith, less than one arc-minute at 45 degrees apparent altitude. However, it increases as altitude decreases, becoming of 35.4 arc-minutes at the horizon [11]. So a start slightly below the horizon is still visible due to the atmospheric refraction.

As a conclusion, we can imagine that people started to construct the hill as an astronomical platform, to have a free horizon for the observation of Alpha Crucis. As time passed, the star was lower and closer to the horizon (or even below it), requiring a higher platform. This could explain the several stages of the building of Silbury Hill.

 

 

Figure 1: Silbury Hill in a Google Earth view.

 

Figure 2: The stars of Crux-Centaurus (Acrux is Alpha Crucis) in 3000 BC as given by Stellarium for the Silbury Hill location. The white line is the horizon.

 

Figure 3: The stars of Crux-Centaurus in 2750 BC as given by Stellarium. The white line is the horizon. Acrux is just below the horizon. It was still visible due to the atmospheric refraction.

 

Figure 4: Acrux is below the horizon in 2500 BC as given by Stellarium. The white line is the horizon.

 

References

[1] Vv. Aa. (2016). Wikipedia, https://en.wikipedia.org/wiki/Silbury_Hill

[2] Atkinson, R. J. C. (1967). Further radiocarbon dates for Stonehenge. Antiquity, 41(161), 63-64.

[3] Meirion Jones, A. (2012). Prehistoric Materialities: Becoming Material in Prehistoric Britain and Ireland. OUP.

[4] http://www.english-heritage.org.uk/visit/places/silbury-hill/

[5] Mackie, E. W. (1977). Science and Society in Prehistoric Britain, New York, St. Martin's Press.

[6] Burl, A. (1979). Prehistoric Avebury.  Yale University press.

[7] Barret, J. (1994). Fragments from Antiquity: An archaeology of social life in Britain 2900-1200BC. Blackwell, Oxford.

[8] Dames, M. (1976). The Silbury Treasure Thames & Hudson Ltd, London.

[9 Leary, J., Field, D. (2010). The Story Of Silbury Hill, English Heritage, Swindon.

[10] Magli, G. (2013). Sirius and the project of the megalithic enclosures at Gobekli Tepe. arXiv preprint arXiv:1307.8397.

[11] Hoskin, M. 2001. Tombs, temples and their orientations, Ocarina books, Bognor Regis.

[12] Allen, C.W. (1976). Astrophysical quantities. London. Athelone Press.

 

 



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This Article was published on 14th June, 2016 at 18:12:56 and has been viewed 712 times.

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The full citation for this Article is:
Sparavigna, A. (2016). A possible role of Alpha Crucis in the astronomical landscape of Silbury Hill. PHILICA.COM Article number 616.


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1 Author comment added 14th June, 2016 at 20:39:53

After publishing the paper, I have found this book “Avebury Cosmos: The Neolithic World of Avebury henge, Silbury Hill, West Kennet long barrow, the Sanctuary & the Longstones Cove”, by Nicholas Mann, John Hunt Publishing, Jun 16, 2011.
The author is discussing a link between Acrux and the Hill and gives a remarkable interpretation of it. “Silbury Hill with its remarkable springs appears as the primal mound, the multi-tiered World Mountain, the ultimate symbol appropriate for a world centre and axis mundi between the worlds. Silbury Hill crowned the horizon in the place where the stars of the Southern Pole had entered the Underworld”.


2 Author comment added 15th June, 2016 at 14:16:28

The reader is invited to see the new version of this paper at the following link:
http://www.philica.com/display_article.php?article_id=617

In the new version, some references had been added. The Mann’s book was properly acknowledged. A new figure showing the sky in the south direction is given. Misprints have been removed.


3 Author comment added 28th June, 2016 at 17:55:48

The reader is invited to see a new version of this paper at the following link:
http://www.philica.com/display_article.php?article_id=630

In the new version, some references had been added. Moreover, the effect of atmosphere on the visibility of stars is simulated using Stellarium software.


4 Author comment added 16th November, 2016 at 08:03:20

The reader are invited to read the new version

http://philica.com/display_article.php?article_id=872

In it I estimated the visual angle gained from the top of the Hill and compared it with the precessional effect.




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