One of the most endearing characteristics of archaeoastronomy is its capacity to set academics in different disciplines at loggerheads with each other.
For a long time I have believed that such diversity requires the invention of some all-embracing theory. I think I was very naïve in thinking that such a thing was ever possible.
Although different ways to do science and different scientific results do arise in different cultures, this provides little support for those who would use such differences to question the sciences’ ability to provide reliable statements about the world in which we live.
At Stonehenge in England and at Carnac in France, in Egypt and Yucatán, across the whole face of the earth, are found mysterious ruins of ancient monuments, monuments with astronomical significance… They mark the same kind of commitment that transported us to the moon and our spacecraft to the surface of Mars.
Archaeoastronomy (also spelled archeoastronomy) is the study of how people in the past “have understood the phenomena in the sky how they used phenomena in the sky and what role the sky played in their cultures.”Clive Ruggles argues it is misleading to consider archaeoastronomy to be the study of ancient astronomy, as modern astronomy is a scientific discipline, while archaeoastronomy considers other cultures’ symbolically rich cultural interpretations of phenomena in the sky. It is often twinned with ethnoastronomy, the anthropological study of skywatching in contemporary societies. Archaeoastronomy is also closely associated with historical astronomy, the use of historical records of heavenly events to answer astronomical problems and the history of astronomy, which uses written records to evaluate past astronomical practice.
Archaeoastronomy uses a variety of methods to uncover evidence of past practices including archaeology, anthropology, astronomy, statistics and probability, and history. Because these methods are diverse and use data from such different sources, the problem of integrating them into a coherent argument has been a long-term issue for archaeoastronomers.
Archaeoastronomy fills complementary niches in landscape archaeology and cognitive archaeology. Material evidence and its connection to the sky can reveal how a wider landscape can be integrated into beliefs about the cycles of nature, such as Mayan astronomy and its relationship with agriculture. Other examples which have brought together ideas of cognition and landscape include studies of the cosmic order embedded in the roads of settlements.
Archaeoastronomy can be applied to all cultures and all time periods. The meanings of the sky vary from culture to culture; nevertheless there are scientific methods which can be applied across cultures when examining ancient beliefs. It is perhaps the need to balance the social and scientific aspects of archaeoastronomy which led Clive Ruggles to describe it as: “…[A] field with academic work of high quality at one end but uncontrolled speculation bordering on lunacy at the other.”
To calculate what astronomical features a structure faced a coordinate system is needed. The stars provide such a system. If you were to go outside on a clear night you would observe the stars spinning around the celestial pole. This point is +90° if you are watching the North Celestial Pole or −90° if you are observing the Southern Celestial Pole. The concentric circles the stars trace out are lines of celestial latitude, known as declination. The arc connecting the points on the horizon due East and due West (if the horizon is flat) and all points midway between the Celestial Poles is the Celestial Equator which has a declination of 0°. The visible declinations vary depending where you are on the globe. Only an observer on the North Pole of Earth would be unable to see any stars from the Southern Celestial Hemisphere at night (see diagram below). Once a declination has been found for the point on the horizon that a building faces it is then possible to say whether a specific body can be seen in that direction.
While the stars are fixed to their declinations the Sun is not. The rising point of the Sun varies throughout the year. It swings between two limits marked by the solstices a bit like a pendulum, slowing as it reaches the extremes, but passing rapidly through the mid-point. If an archaeoastronomer can calculate from the azimuth and horizon height that a site was built to view a declination of +23.5° then he or she need not wait until 21 June to confirm the site does indeed face the summer solstice. For more information see History of solar observation.
The Moon’s appearance is considerably more complex. Its motion, like the Sun, is between two limits — known as lunastices rather than solstices. However, its travel between lunastices is considerably faster. It takes a sidereal month to complete its cycle rather than the year long trek of the Sun. This is further complicated as the lunastices marking the limits of the Moon’s movement move on an 18.6 year cycle. For slightly over nine years the extreme limits of the moon are outside the range of sunrise. For the remaining half of the cycle the Moon never exceeds the limits of the range of sunrise. However, much lunar observation was concerned with the phase of the Moon. The cycle from one New Moon to the next runs on an entirely different cycle, the Synodic month. Thus when examining sites for lunar significance the data can appear sparse due the extremely variable nature of the moon. See Moon for more details.
Finally there is often a need to correct for the apparent movement of the stars. On the timescale of human civilisation the stars have maintained the same position relative to each other. Each night they appear to rotate around the celestial poles due to the Earth’s rotation about its axis. However, the Earth spins rather like a spinning top. Not only does the Earth rotate, it wobbles. The Earth’s axis takes around 25,800 years to complete one full wobble. The effect to the archaeoastronomer is that stars did not rise over the horizon in the past in the same places as they do today. Nor did the stars rotate around Polaris as they do now. In the case of the Egyptian pyramids, it has been shown they were aligned towards Thuban, a faint star in the constellation of Draco. The effect can be substanstial over relatively short lengths of time, historically speaking. For instance a person born on 25 December in Roman times would have been born with the sun in the constellation Capricorn. In the modern period a person born on the same date would have the sun in Sagittarius due to the precession of the equinoxes.
Newgrange is a passage tomb in the Republic of Ireland dating from around 3,300 to 2,900 BC For a few days around the Winter Solstice light shines along the central passageway into the heart of the tomb. What makes this notable is not that light shines in the passageway, but that it does not do so through the main entrance. Instead it enters via a hollow box above the main doorway discovered by Michael O’Kelly. It is this roofbox which strongly indicates that the tomb was built with an astronomical aspect in mind. Clive Ruggles notes:
“Few people - archaeologists or astronomers- have doubted that a powerful astronomical symbolism was deliberately incorporated into the monument, demonstrating that a connection between astronomy and funerary ritual, at the very least, merits further investigation.”
The Pyramids of Giza
Since the first modern measurements of the precise cardinal orientations of the pyramids by Flinders Petrie, various astronomical methods have been proposed for the original establishment of these orientations. It was recently proposed that this was done by observing the positions of two stars in the Plough / Big Dipper which was known to Egyptians as the thigh. It is thought that a vertical alignment between these two stars checked with a plumb bob was used to ascertain where North lay. The deviations from true North using this model reflect the accepted dates of construction.
Some have argued that the pyramids were laid out as a map of the three stars in the belt of Orion, although this theory has been criticized by reputable astronomers.
El Castillo, also known as Kukulcán’s Pyramid, is a Mesoamerican step-pyramid built in the centre of Mayan center of Chichen Itza in Mexico. Several architectural features have suggested astronomical elements. Each of the stairways built into the sides of the pyramid has 91 steps. Along with the extra one for the platform at the top, this totals 365 steps, which is possibly one for each day of the year (365.25) or the number of lunar orbits in 10,000 rotations (365.01). A visually striking effect is seen every March and September as an unusual shadow occurs on the equinoxes. A shadow appears to descend the west balustrade of the northern stairway. The visual effect is of a serpent descending the stairway, with its head at the base in light. Additionally the western face points to sunset around 25 May, traditionally the date of transition from the dry to the rainy season.
Many astronomical alignments have been claimed for Stonehenge, a complex of megaliths and earthworks in the Salisbury Plain of England. The most famous of these is the midsummer alignment, where the Sun rises over the Heel Stone. However, this interpretation has been challenged by some archaeologists who argue that the midwinter alignment, where the viewer is outside Stonehenge and sees the sun setting in the henge, is the more significant alignment, and the midsummer alignment may be a coincidence due to local topography. As well as solar alignments, there are proposed lunar alignments. The four station stones mark out a rectangle. The short sides point towards the midsummer sunrise and midwinter sunset. The long sides if viewed towards the south-east, face the most southerly rising of the moon. Aveni notes these have never gained the acceptance which the claims solar alignments have. Jacobs noted the Heel Stone azimuth is one-seventh of circumference, matching the latitude of Avebury, while summer solstice sunrise azimuth is no longer equal to the construction era direction.
This is an architecturally outstanding Neolithic chambered tomb on the Mainland of Orkney – probably dating to the early 3rd millennium BC, and where the setting sun at midwinter shines down the entrance passage into the central chamber (see Newgrange). In the 1990s further investigations were carried out to discover whether this was an accurate or an approximate solar alignment. Several new aspects of the site were discovered. In the first place the entrance passage faces the hills of the island Hoy, about 10 miles away. Secondly, it consists of two straight lengths, angled at a few degrees to each other. Thirdly, the outer part is aligned towards the midwinter sunset position on a level horizon just to the left of Ward Hill on Hoy. Fourthly the inner part points directly at the Barnhouse standing stone about 400m away and then to the right end of the summit of Ward Hill, just before it dips down to the notch between it at Cuilags to the right. This indicated line points to sunset on the first Sixteenths of the solar year (according to A. Thom) before and after the winter solstice and the notch at the base of the right slope of the Hill is at the same declination. Fourthly a similar ‘double sunset’ phenomenon is seen at the right end of Cuilags, also on Hoy; here the date is the first Eighth of the year before and after the winter solstice, at the beginning of November and February respectively – the Old Celtic festivals of Samhain and Imbolc. This alignment is not indicated by an artificial structure but gains plausibility from the other two indicated lines. Maeshowe is thus an extremely sophisticated calendar site which must have been positioned carefully in order to use the horizon foresights in the ways described.
Uxmal is a Mayan city in the Puuc Hills of Yucatán, Mexico. The Governor’s Palace at Uxmal is often used as an exemplar of why it is important to combine ethnographic and alignment data. The palace is aligned with an azimuth of 118º on the pyramid of Cehtzuc. This alignment is also towards a southerly rising of Venus which occurs once every eight years. By itself this would not be sufficient to argue for a meaningful connection between the two events. The palace has to be aligned in one direction or another and why should the rising of Venus be any more important than the rising of the Sun, Moon, other planets, Sirius et cetera? The answer given is that not only does the palace point towards the rising of Venus, it is also covered in glyphs which stand for Venus and Mayan zodiacal constellations. It is the combination of the alignment and the ethnography which suggests that the city was built with cosmic order in mind.