Abstract: The paper is proposing several examples of the use of Stellarium, an open source free software planetarium, for simulating the astronomical ...
The Stellarium Planetarium for the Simulation of the Astronomical Landscapes of Ancient Stargazers Amelia Carolina Sparavigna Politecnico di Torino Abstract: The paper is proposing several examples of the use of Stellarium, an open source free software planetarium, for simulating the astronomical landscapes of the ancient stargazers. Here we will focus on the motion of the stars, in particular with respect to the precession of Equinoxes. Usually, we consider that this precession was discovered by Hipparchus of Nicaea (c. 190 – c. 120 BC), Greek astronomer and mathematician, who measured it comparing the positions of the stars that he observed to the data recorded a hundred of years before. In fact, Hipparchus gave us the first historical record of the precession but probably he was not the first to observe this motion of the heaven of the fixed stars. We can argue this possibility from the simulation of the sky seen by the ancient stargazers. Keywords: Archaeoastronomy, Silbury Hill, Carnac Stones, Babylon, Pyramids of Giza, Gobekli Tepe, Alpha Crucis, Vega, Rigil Kent, Fomalhaut, Capella, Canopus, North Star, Sirius, Orion, Deneb.
Introduction In this paper we will show several examples of the use of Stellarium, an open source free software planetarium, for simulating the astronomical landscapes of the ancient stargazers. Here we will focus on the motion of the stars, or, more specifically of the “heaven of the Fixed Stars” that seems to turn about the “axis mundi”, the axis about which the universe (mundus) appears to rotate, and on the effect of the precession of the Earth’s axis on this motion. The precession of which we are talking is the precession of the Equinoxes, that is, it is the axial precession of the Earth. Here we do not describe it is details, suggesting the reading of the following link of Wikipedia: https://en.wikipedia.org/wiki/Axial_precession . Usually, we consider that this precession was discovered by Hipparchus of Nicaea (c. 190 – c. 120 BC), Greek astronomer, geographer, and mathematician. He measured the precession comparing the observed positions of the stars to the data recorded a hundred of years before. In fact, we have from Hipparchus the first historical record of the precession but probably he was not the first to observed that the motion of the stars had a change during the centuries. We can argue this possibility from the simulation of the sky seen by the ancient stargazers. We will consider locations in England, France, Mesopotamia, Egypt and Turkey, the choice of which will be evident by the discussion. However, before proposing the examples, some discussions are necessary. Natural hills for observing the sky The fact that an elevated position can help an observer to see the sky and the stars near the horizon is obvious. Less obvious is the evaluation of the distances and the dip of the horizon we can see when we change our height with respect to the ground. Formulas and explanations are available at the following links. At https://en.wikipedia.org/wiki/Horizon we have an illustration of the different horizons: astronomical, visible and true. In the previous link and in the following, some formulas and calculations are necessary (examples are given too). The other link is https://en.wikibooks.org/wiki/Trigonometry/The_distance_and_dip_of_the_horizon. For the dip of the horizon, see also this page http://aty.sdsu.edu/explain/atmos_refr/dip.html by Andrew Young. Let us simplify the approach. To evaluate the dip of the horizon, that is, the angle below the astronomical horizon that we have if we change our height, let us use the formula for the dip D we find in the Reference [1]. If h is the height of the eyes in meters, we have that the dip D of the horizon
Electronic copy available at: https://ssrn.com/abstract=2872676
is D = 1.78 sqrt(h), the result is in minutes of arc (in fact, the dip is an angle). This formula takes into account the normal refraction of the atmosphere. If we use this formula for h = 40 m, we find an angle α = 0.18 degrees. In the case of a height of 400 m, α = 0.59 degrees. To appreciate the value of this angle, let us remember that the apparent size of the Moon is about 0.5 degrees. Therefore, in http://www.photopills.com, we find the following warning. “When you are at the top of a mountain or on the roof of a tall building, at a considerable height above the horizon, you need to adjust the calculations of the sun/moon rise/set times. For example, if an Observer is located at the top of a mountain, at 2.000 feet (about 600 m) of height above his horizon, he will see the sunrise earlier than other observer located at the horizon level.” In fact, we have to consider this last level as the level of the astronomical horizon. What told for sun and moon is true for the stars too. Then, if a star is below the horizon for an observer at the sea level, it can be visible, if it is enough bright, to an observed on a hill. We can easily imagine that the first stargazers used some hills to observe the sky and have a free horizon, with reference points such as distant hills or mountains, for monitoring the rise and setting of stars according to the seasons. Some specific natural places were so good for observations that assumed also a spiritual relevance. Silbury Hill One of the most famous archaeological site in the world is Stonehenge, which is linked with several other nearby sites such as Avebury and the Silbury Hill, a prehistoric artificial mound. Of the Hill, I discussed in [2]. As proposed by Nicholas Mann [3], the Hill is linked to the disappearing of the stars of the Crux (about this constellation, https://en.wikipedia.org/wiki/Crux ) from the Avebury astronomical landscape, due to the precession of the Earth’s axis. The Hill was a “mountain” above the South Pole imagined to support the falling stars [3]. In [2], I proposed it as a platform for astronomical observation, the height of which could have compensated, for a certain period, the effect of precession. In fact, about 2750 BC, Alpha Crucis was disappearing below the horizon. The Hill is 40 meters high. This is a very interesting fact, because an observer on it can see a free astronomical horizon, and for some directions, even a dip of the horizon of about 0.18 degrees. Before the simulations of the astronomical landscape, let us point out that the astronomical horizon is that given by using Stellarium (http://www.stellarium.org). This software seems not including the dip of the horizon. However, we can consider it as a correction of the results we have from the software. Stellarium is fundamental to have a representation of the astronomical landscapes, because it is considering the axial precession in the representation of the sky. Then let us start with examples. Silbury Hill, due South, 4000 BC. In the following image, we see the white line representing the horizon. The grid is that of the azimuthal frame.
Electronic copy available at: https://ssrn.com/abstract=2872676
The grid is that of the azimuthal frame of reference, the size being of 10 degrees (so it is difficult to appreciate a shift of the horizon of 0.18 degrees, that is about 1/3 of the apparent size of the Moon). In the image we find that people saw the stars of the Crux, with the bright Acrux (Alpha Crucis). In 4000 BC, Alpha Crucis was about 5 degrees above the horizon. Silbury Hill, due South, 3000 BC.
Here we have the effect of precession. The Crux is lower. Acrux is above the astronomical horizon.
3000 BC, simulation without atmosphere and simulation with atmosphere. It is important to note that we have the effect of the atmospheric refraction and absorption. We can simulate these effects in Stellarium (previous images). The refraction of atmosphere is moving the apparent position of the stars near the horizon. Then, a star could be below the horizon, but visible due to the refraction. However, for being visible, this star must be bright enough (such as Alpha Crucis). In the following image, another portion of the sky, where Orion and Sirius are visible.
Let us observe the sky due North. It is interesting to observe that Vega was circumpolar in 3000 BC. In 2000 BC it was moving closer the horizon. Silbury Hill, due North, 3000 BC.
Silbury Hill, due North, 2000 BC
A curiosity. Today (18 November 2016), from Silbury Hill we can see Vega on the horizon, as shown in the following image. The effect of atmosphere is visible in a further simulation. Silbury Hill, today, due North
Silbury Hill, today, due North (with atmosphere)
Carnac Stones The Carnac stones are an exceptionally dense collection of megalithic sites, many around Carnac and some within La Trinité-sur-Mer, in France, consisting of alignments, dolmens, tumuli and single menhirs. As told in https://en.wikipedia.org/wiki/Carnac_stones we find more than 3,000 standing stones, erected by the pre/proto-Celtic people of Brittany. The stones were erected at some stage during the Neolithic period, probably around 3300 BC. The erection of some of them may date to as early as 4500 BC. For a link of the stones to the sky, we have seen that one of the site, the “quadrangle” is connected to solstices and lunar standstills [4]. However, what was the astronomical landscape seen by the people of Carnac? Here some examples. Carnac Stones, due South, 4500 BC
Here we can see the Crux and the Rigil Kent. The form “Rigel Kent” or Rigel Kentaurus is often used as an alternative name of Alpha Centauri. This is the closest star system to the Solar System at a distance of 4.37 light-years. It consists of three stars: the pair Alpha Centauri A and Alpha Centauri B and a small and faint red dwarf, Proxima Centauri. To the unaided eye, the two main stars appear as a single object, forming the brightest star of the Centaurus and the third-brightest star in the night sky, after Sirius and Canopus.
Carnac Stones, due South, 4500 BC, 5000 BC and 4000 BC, Fomalhaut
As we have seen for Silbury Hill, the ancient people had the possibility to observe the effect of precession, seeing some stars disappearing (or appearing, as we will discuss in the following) due South. The ancient stargazers could also see some of the circumpolar stars of the North Celestial Pole became stars that rise and set. In the three figures concerning Fomalhaut, we show that this star was making an arc about South, which was becoming smaller and smaller, passing from an angle of 3 degrees (5000 BC) to 0 degrees in 4000 BC. Of course this is a very long period, but some lore about this star probably existed, about its behavior. The star's traditional name derives from Fom al-Haut, from Arabic, meaning the "mouth of the whale". Carnac Stones, due South, 4500 BC, Orion, Sirius and Aldebaran.
The previous image shows another starry night in Carnac. Let us conclude the examples concerning Carnac with another star. It is Capella announcing the sun rising due East at an equinox. Carnac Stones, 4500 BC, Capella announcing the sunrise due East at an equinox.
Babylon Let us change latitude and move to Babylon. It was a major city of ancient Mesopotamia in the plain between the Tigris and Euphrates rivers. Wikipedia, https://en.wikipedia.org/wiki/Babylon, is telling that the city was built upon the Euphrates and in origin was a small Akkadian city dating from the period of the Akkadian Empire c. 2300 BC. Babylon had a ziggurat, Etemenanki, In Sumerian, the “temple of the foundation of heaven and earth", dedicated to Marduk. Originally the ziggurat was 91 meters in height. Etemenanki is considered a possible inspiration for the biblical story of the Tower of Babel. In https://en.wikipedia.org/wiki/Etemenanki, we have that “it is unclear exactly when Etemenanki was first built. A review article by Andrew R. George says that its builder may have "reigned in the fourteenth, twelfth, eleventh or ninth century [BC]" but argues that the reference to a ziqqurrat at Babylon in the Creation Epic, is supporting the long-held theory that it existed already in the second millennium BC. [5] The ziggurat of Babylon was 91 meters height, much more than the Silbury Hill. The dip of the horizon is 0.28 degrees (about ½ of the moon apparent dimension). We could ask ourselves if there was any star requiring a specific astronomical observation, like Alpha Crucis in the landscape of Silbury Hill. Let us start our simulations using Stellarium. Ziggurat of Babylon, due South, 4000 BC and 3000 BC (Crux and Rigil Kent)
Was Alpha Crucis? As we can see from the previous images the answer is negative. The Crucis is well above the horizon. But another star exists, quite interesting and this star is Canopus, that is, Alpha Carinae, the second brightest star in the night-time sky, after Sirius. The name of the star is considered as originated from the mythological Canopus, who was a navigator for Menelaus, king of Sparta. But, as told in https://en.wikipedia.org/wiki/Canopus and references therein, Canopus was not visible to the ancient Greeks and Romans. It was visible to the ancient Egyptians. Ziggurat of Babylon, due South 4000 BC, 3000 BC and 2000 BC, Canopus
Due to precession, Canopus, that was below the horizon, became visible about 3000 BC. This star was therefore indicating the South direction. To understand the different behavior of the Crux and Canopus, with respect to precession, let us see them in the same image. Here the sky in 4000 BC. In the image there are both azimuthal and horizontal grids.
In the following image, the sky in 2000 BC. The Crux moved towards the South Celestial Pole. Canopus moved away.
Sky in 2000 BC (Babylon)
Let us turn the simulation due North. Note the behavior of the North Star (Stella Polare). Ziggurat of Babylon, due North, 4000 BC and 2000 BC.
Let us conclude this part of the article devoted to the ziggurat of Babylon, proposing that Canopus, the new bright star appearing above the horizon about 3000 BC, had a relevant role for the local people. Pyramids of Giza. As previously told, Canopus was visible in Egypt. Let us consider the sky as we can see from the well-known Pyramids of Giza. The oldest of them is the Great Pyramid, also known as the Pyramid of Khufu (from the top of the pyramid, 139 m height, the dip is of 2/3 of the apparent size of the Moon. As explained by https://en.wikipedia.org/wiki/Great_Pyramid_of_Giza, “based on a mark in an interior chamber naming the work gang and a reference to fourth dynasty Egyptian Pharaoh Khufu, Egyptologists believe that the pyramid was built as a tomb over a 10 to 20-year period concluding around 2560 BC”. Canopus was visible from Giza as early as 4000 BC.
We can also see the sky due North. The following map gives it about 3000 BC.
Giza, 3000 BC, North Celestial Pole.
For the ancient Egyptians, a very important star was Sirius. This is the brightest star in the Earth's night sky, twice as bright as Canopus, the next brightest star. The name "Sirius" is derived from the Ancient Greek Σείριος (Seirios), meaning "the glowing one". In ancient Egypt, the star was Sopdet (Greek: Σῶθις Sothis). During the Middle Kingdom, Egyptians based their calendar on the heliacal rising of Sirius, that occurred just before the annual flooding of the Nile and the summer solstice. For other historical information, see please https://en.wikipedia.org/wiki/Sirius. Sirius appeared on the Giza horizon about 12000 BC. Giza, 12000 BC, due South, Sirius appears above the horizon.
Sirius, Orion, Deneb and Gobekli Tepe Sirius could have influenced also other populations. In [6], the megalithic enclosures of Gobekli Tepe in Turkey are linked to the appearance on the local horizon of Sirius. For what concerns the site, in https://en.wikipedia.org/wiki/Göbekli_Tepe, we find that a stratigraphy of it attests to many centuries of activity, beginning at least as early as the epipaleolithic period (a period from 18000 BC to 8500 BC). Structures identified with the succeeding period, Pre-Pottery Neolithic A (PPNA), have been dated to the 10th millennium BCE. Remains of smaller buildings identified as Pre-Pottery Neolithic B (PPNB) and dating from the 9th millennium BCE have also been unearthed. Magli linked some of these structures to the rising of Sirius above the horizon. Gobleki Tepe, due South, 9000 BC, Sirius.
The site http://www.bibliotecapleyades.net/arqueologia/gobekli_tepe08.htm by Andrew Collins is proposing an interesting “Battle of the Stars”. On the summit of a mountain ridge, with clear views of the local horizon – the page is telling - it makes sense to explore the possibility that the Göbekli Tepe’s megalithic structures were linked to the stars. This attracted several studies. In 2012 Robert Schoch, University of Boston, proposed that Göbekli Tepe was linked to the rising of the belt stars of Orion, “as they would have appeared on the horizon during the epoch of their construction”. After, Giulio Magli, University of Milano, proposed in [6] the link to the Sirius rising. The engineer Rodney Hale analyzed this hypothesis. “He determined that when Sirius first began to rise again around 9500 BC, the star only managed a feeble arc across the southern horizon before disappearing out of sight, a situation that barely changed for hundreds of years.” This happens because the nearer a star is to the horizon, the dimmer it will appear to the eye due to atmospheric. Hale concluded therefore that Sirius was a “somewhat unimpressive star” for people of Göbekli Tepe. The site http://www.bibliotecapleyades.net/arqueologia/gobekli_tepe08.htm continues in the following manner. “Schoch and Magli had concentrated their efforts in identifying a target star in the southern sky, simply because the stone structures at Göbekli Tepe are located in the southern section of its occupational mound, suggesting that the gaze of its assumed astronomer-priests was toward the south. Yet there is every reason to believe that the structures are directed not southwards, towards Orion or Sirius, but north towards the circumpolar (i.e., never setting) and near circumpolar stars that forever turn about the celestial pole.” In fact, Rodney Hale, using surveys of the enclosure, determined “that just one bright star aligned with the portholes each night (images of the holes are
given in http://www.bibliotecapleyades.net/arqueologia/gobekli_tepe08.htm) and this was Deneb in the Cygnus Constellation, the celestial swan, also known as the Northern Cross. This meant that if a person were to stand between the twin central pillars in these enclosures, they could have watched Deneb set through the circular apertures of the holed stones.” Gobleki Tepe, due North, 9600 BC.
Gobleki Tepe, due North, 9000 BC.
After 9500 BC, due to precession, Deneb was no more a circumpolar star. It was rising and setting, as we can see simulating the year 9000 BC. “So why might the Göbekli builders have been interested in this particular star? – continues http://www.bibliotecapleyades.net/arqueologia/gobekli_tepe08.htm - The answer lies in the fact that it marks the point on the Milky Way where it splits to form two separate streams, due to the presence of stellar dust and debris in line with the axis of the galactic plane. Ancient cultures saw this fork or cleft, known to astronomers as the Great Rift or Cygnus Rift, as an entrance to the sky-world, or upper world, existing beyond the physical realm, an idea that might well go back to Paleolithic times.” In any case, due South or due North, people of Gobleki Tepe could see that something in the sky was changing. Conclusion In this paper we have shown several examples of the use of Stellarium, in particular to evidence the effect of precession in the archaeoastronomical analyses. In the software we can set coordinates and time of observations. The software is also simulating the presence of the atmosphere. Stellarium is a software very easy to use. Let us remember that the height of the observer is relevant to analyze the stars when they are moving close to the horizon, in particular when moving near the South direction. The height of the observer in fact, is changing the dip of the horizon. Therefore, in any simulation made by means of Stellarium, which is giving the astronomical horizon, a correction can be made by the formula given in the first line of the second page. References [1] Freiesleben, H. C. (1954). The dip of the horizon. Navigation, 4(1), 8-9. [2] Sparavigna, A. C. (2016). A Possible Role of Alpha Crucis in the Astronomical Landscape of Silbury Hill. PHILICA, Article 872, 2016. Available at SSRN: https://ssrn.com/abstract=2795708 [3] Mann, N. (2011). Avebury Cosmos: The Neolithic World of Avebury henge, Silbury Hill, West Kennet long barrow, the Sanctuary & the Longstones Cove, John Hunt Publishing. [4] Sparavigna, A. C. (2016). Megalithic Quadrangles and the Ancient Astronomy (November 1, 2016). Available at SSRN: https://ssrn.com/abstract=2862330 or http://dx.doi.org/10.2139/ssrn.2862330 [5] George , Andrew (2007). The Tower of Babel: Archaeology, history and cuneiform texts. Archiv für Orientforschung, 51 (2005/2006). pp. 75-95. [6] Magli, G. (2013). Sirius and the project of the megalithic enclosures at Gobekli Tepe. Nexus Network Journal, 1-10.
