The Tangardak Kaya cave is formed in a separate Karst rock massif, close to the ridge of the Ilinitza elevation. The rocks are Cretaceous limestone. They have been subject to the strong influence of the endogenous forces (tectonic movements). The processes of physical and chemical weathering have led to the formation of an entrance � a widened tectonic fissure in the base of which there are several small terraces. Later, following the main tectonic fissure, an artificial gallery was created. The gallery is 12 m long and has an average width of 1.5 m. In the foundation, following the fissure a very sloped corridor had been created. This corridor leads to the entrance of the cave. At the cave�s ground level the gallery�s entrance opening is widened in its foundation. The vertical section, looked at from the inside, has the form of the ellipse with a big eccentricity. The cave�s bottom had been formed especially to fulfill obviously some altar functions. This bottom was hewn in the rock as a small terrace.
Also, there are two neighbour zones with almost elliptic section in the middle of the gallery. The gallery�s ceiling has a clearly expressed vault form. One registers sound increase as well as significant reverberation (a loud noise repeated as an echo) and very long echo in the focuses of the vaults. Maximal increase and reverberation time are in the region of lower sound frequencies. The astronomical azimuth of the cave�s main axis is A = 15° 08� 12��. Analysis of the cave�s horizontal plan shows that the main axis follows the development of the tectonic fissures in the Karst massif.
The clearly expressed orientation of the cave�s main axis following the main meridian (North-South), as well as the quality of the entry opening to project itself maximally along the gallery at noon, gives one a reason to seek astronomical meaning in the geometric sizes, orientation and morphology expressed in its main spatial characteristics. In this light, except for the exact location of the "Tangardak Kaya" cave, one has to know the exact azimuths of the most significant astronomical phenomena of the relevant geographical latitude of the site. This is important especially regarding the Sun�s and Moon�s rises, culminations and sets. The most important natural phenomena which determine a person�s entire life, consequently all defining characteristics of the material and spiritual culture of the ancient population, are related to the astronomical landmarks.
The observers of the celestial sphere in the Antiquity could not follow the rises and the sets of the brightest of luminaries (the Sun, the Moon, bright planets and stars) from the "Tangardak Kaya" cave due to the limited visibility of the line of the local horizon. Making use of the natural orientation of the tectonic fissure, the people positioned the main axis of the cave almost in the plane of the main meridian. This is related with the search of deliberate orientation as well as with the desire of the ancient builders to follow the line of the least resistance of the medium. With the resulting artificial volume, the observer could note the moments of the luminaries� upper culmination. The maximal length of the projection of the illuminated entrance opening showed noon.
Researching the astronomical circumstances of these culminations, one notices the fact that projections of the entrance opening during the summer are short and move only at the entrance of the cave. The long projections can be realized only when the Sun has negative declinations during the winter season. Using the data from the Permanent part of the Astronomical calendar, one calculates the Sun�s maximal height during its upper and lower culmination.
The equation hmax = 90 - φ + ε is executed in the point of the summer solstice (where the solar declination δ is at its maximum, and equals the slope of the ecliptic ε. For the contemporary epoch, for which ε = 23° 26� 24", the distance between the peak of the entrance projection and the altar�s foundation is 10,60 m. In the point of the winter solstice the solar declination δ is negative and at its maximum, and equals to -ε. The equation hmin = 90 - φ - ε is executed and the distance between the peak of the entrance projection and the altar�s foundation is 1,10 m. Considering that the astronomical azimuth A of the cave�s main axis is approximately 15°, the height of the Sun will decrease with one more degree (1°). Consequently, the projection will come 0,25 m closer to the altar�s foundation. Sun culminates high above the horizon during the summer and the higher outer contour of the entrance opening is projected on the floor of the gallery. During the winter, when the Sun culminates at lower heights, the lower inner contour of the entrance opening is projected.
The ecliptic�s slope decreases with time. This means that in the past the solar height at noon during the winter solstice would have become smaller and smaller; meaning the light from the entrance�s projection would have crept closer and closer to the altar. For example, between 2000 B.C. and 1000 B.C. the projection of the entrance opening did reach up to 0.4 m from the altar�s foundation. Table 1 represents the change of the ecliptic�s slope from 4000 B.C. to 0 B.C. and the solar height at noon at the point of the winter solstice.
|Years B.C.||Slope of the ecliptic ε||Solar height at noon at the point of the winter solstice|
|4000||24° 06� 36��||24° 10� 36��|
|3500||24° 04� 12��||24° 13� 00��|
|3000||24° 01� 48��||24° 15� 24��|
|2500||23° 58� 48��||24° 18� 24��|
|2000||23° 55� 48��||24° 21� 24��|
|1500||23° 52� 12��||24° 25� 00��|
|1000||23° 48� 36��||24° 28� 36��|
|500||23° 44� 24��||24° 32� 48��|
|0||23° 40� 12��||24° 37� 00��|
The results of this research demonstrate that this cave-sanctuary can be related with the profession of the cult to the Great Goddess-Mother � once per year the solar ray penetrates the altar, embodying her sacred marriage with the Sun (Fol V., 2000). Besides, the sanctuary Tangardak Kaya could serve as an instrument for determining the year�s length and its beginning with sufficient accuracy. Systematic observations of the positions of the entrance projections during the daily solar culminations allows one to count the days between the winter and the summer solstice. This procedure would greatly facilitate the creation and usage of a primitive calendar and the measurement of time with units larger than a day. This usage is intrinsically related with the economic, religious and daily requirements of the socium of that epoch.
Authors: Alexey Stoev, Penka Muglova.
1. Fol, V. 2000. Megalithic and Rock-cut monuments in Ancient Thrace. Sofia University Press "St. Kl. Ohridski" & Demax, Sofia. /in Bulg./
2. STOEV, A. AND OTHERS, 1997, Research on drawings representing celestial phenomena and cosmological elements from cave sanctuary from the Neolithic. Proceedings of the 12th International Congress of Speleology, vol. 3, La Chaux de Fonds, Switzerland, 95-96.
3. STOEV, A. AND OTHERS, 2001, Spatial orientation and acoustic reverberation in the Tangardak Kaya cave sanctuary near the village of Ilinica, Kardgali District. Proceedings of Perperek I, New Bulgarian University, Sofia, 226-325 /in Bulg./