Archaeo-astronomy tries to find indications of astronomical knowledge and activities from archaeological, unwritten evidence of past cultures. Observing the sky can be motivated by the aspect of measuring time as well as for cultic or even purely aesthetic reasons. The most obvious objects of interest are the annual course of the Sun, the monthly motion of the Moon and its remarkable 19-year periodic recurrence known as the Metonic cycle, and possibly the daily and annual changes in the fixed star sphere. The planets can reach a too wide range of positions in the sky, and are only rarely considered, except for the 8-year periodic recurrence of Venus. A typical field of research is the orientation of cultic buildings, e.g., temples. Examples are the Pyramids of Egypt, the temple in Persepolis [Schlosser and Cierny 1996, p.109ff], and Newgrange and Stonehenge as best known stone-age sites.
Our seasons are defined to start at the equinoxes and solstices, where the Sun rises due east, or at the northermost or southernmost position, respectively. Other cultures defined the seasons not as beginning and ending at solstices and equinoxes, but centered the seasons around these dates [Krupp 1994]; this practice leads to terms like “midsummer”, which is still used for summer solstice. In total, the Sun thus provides eight calendrical key dates with five characteristic declinations, which leads to ten characteristic azimuths, some or all of which should be traceable in buildings classified as “solar oriented”.
The Moon can likewise reach a wide range of declinations. Due to its orbit being slightly tilted from the Earth’s orbital plane, its extreme north and south declinations vary even wider. Thom and Thom [1978] found interesting connections of the orientation of stone circles and rows in Great Britain and Brittany with Moon-related azimuths, and Pavúk and Karlovský [2004] proposed lunar orientations in KGAs of Slovakia.
Fixed stars pose a special problem in archaeo-astronomy: Although called fixed, over the course of centuries and millennia stars do change their position on the celestial sphere mostly due to the precession of the Earth’s rotational axis, but also due to the proper motion of the stars. Therefore, to investigate stellar alignments, at least the approximate age of an archaeological structure must be known. The KGAs all share a similar age and all have been built within about 300 years, providing an excellent sample for such studies. Meeus [1998, Ch.21] describes algorithms to correct stellar catalog positions given for one epoch to another.
From the geographical position of the observer, the object’s celestial coordinates on the imaginary infinite celestial sphere, and the local date and time of observation, it is possible to compute a celestial object’s azimuth, i.e., its horizontal direction, and especially its azimuths of rising and setting, but only on the mathematical horizon [e.g., Meeus 1998].
A crucial problem of all archaeo-astronomical orientation studies is the availability of landscape horizon data. Computing rising or setting directions along the mathematical horizon is simple but useless, because – if at all – any rising of the Sun can only be observed on the true landscape horizon, and thus any entrance or other alignment intended to point into, e.g., a solstice sunrise direction could only have been built taking the landscape horizon into account.
On any day of the year, stars rise and set approximately 4 minutes earlier than on the day before. Stars which surround the celestial pole of the observer’s hemisphere do not set, they are visible in every clear night and are called circumpolar. Others are only visible during certain parts of the year, the Sun being too close to them to be observable the other times. The last day such a star is visible at dusk is called the day of its heliacal setting, where the star becomes visible slightly above the horizon in the evening twilight, only to set moments later or vanish in the horizon haze. After a few weeks of invisibility due to its proximity to the Sun, the star emerges in the morning on the day of its heliacal rising: in the morning twilight, the star appears shortly over the horizon haze, only to become too washed out a few moments later by the increasing brightness of the emerging daylight. On each successive day, the star will be visible about four minutes earlier. It has long been known that heliacal risings of selected stars have been used by various peoples all over the world to mark specific days of the year [Krupp 1994], the best known example being the Egyptian use of the heliacal rising of the sky’s brightest star, Sirius, as “herald” of the Nile flood. While the exact day in each year also depends on atmospheric conditions, the average day can be computed at least approximately [Mucke 1993; Meeus 1997, ch.46; Schaefer 1985, 1987].
Currently, three astronomically related hypotheses exist in parallel for the KGAs:
Such studies require both an accurate survey of the archaeological features, and knowledge of the astronomical processes and stellar positions of the respective period. In addition, an interpretation of the potential use of systematic sky observations aided by the proposed orientation can help to understand the aspects of practical astronomical activities and knowledge of the sites’ users.
Many azimuth directions can be found in prehistoric monuments which provide a star that rises or sets there at some period in time. Therefore, investigations of singular monuments are always prone to the danger of overinterpretation.
In the preliminary study [Zotti forthcoming], all plausible orientations of entrances, radial ditches and palisade gaps have been integrated in a specially designed circular histogram and combined with the diurnal arcs of the sun at the 8 solar key dates, the lunar key declinations, and bright stars. The histogram (Fig. 3) shows many singular entries for entrance or ditch directions, and many stars that rise or set where these directions seem to point on the (mathematical) horizon.
Not every entrance or palisade gap was necessarily intended to be aligned towards some rising or setting point of a certain object. However, the frequency distribution in the histogram shows respective peaks for some entrance directions. Furthermore, it was already possible to propose an interpretation for a few of the detected peaks.
For the directions associated with the Sun, the most obvious peaks are near azimuth 127∘ (sunrise at Winter solstice) and south of 115∘ (sunrise at Candlemas/All-Saints-Day), thus marking start, middle and end of a “Winter” season centered around the Winter solstice. On the northern side, an equally defined “Summer” appears to be marked mostly with begin/end dates, while the summer solstice seems to have been less important.
Still, it seems evident that not only solar directions played a role in the orientations of KGAs. While our data currently do not offer a clear connection to lunar extremal risings or settings (dashed green arcs in fig. 3), there are two strong peaks towards directions 104–108∘ and 275–280∘. Indeed, around 4700 BC, there have been two very conspicuous objects rising respectively setting at these directions: the Pleiades star cluster (rising) and the bright star Antares in Scorpius (setting). Moreover, the events of rising Pleiades and setting Antares took place almost simultaneously.
Further investigation led to the conclusion that the Pleiades’ heliacal rising (see section 2.2) at that epoch took place just after spring equinox. The Pleiades have been observed by many cultures worldwide in connection with the seasons [Krupp 1994], although a star cluster like the Pleiades, which consists of dim stars, cannot be observed below an extinction angle of about aE = 4∘. The current lack of horizon data however impedes primary research on the question whether the Pleiades may have risen visibly behind a raised landscape horizon (e.g., chain of hills).
Along the northern and southern horizon are directions that — if astronomically significant — can only point to rising or setting positions of stars. Unfortunately, the intersections on the northern and southern horizon are very sensitive to elevated horizon lines, so the peaks in the histogram, which cannot contain a horizon line, appear less concentrated. Furthermore, on the southern horizon, there are several stars sharing almost the same declination, so any singular star could not be identified yet.
The research done so far has to be extended and improved, so that current hypotheses concerning astronomical aspects of KGAs can be either falsified or further supported, or new hypotheses may be set up.
