VOLUME XXII 2009 1 © 2009 by the University of Texas Press, P.O. Box 7819, Austin, TX 78713-7819 A Bayesian Approach to the Orientations of Central Alentejo Megalithic Enclosures FERNANDO PIMENTA, LUÍS TIRAPICOS, AND ANDREW SMITH Fernando Pimenta is a member of the Portuguese Association of Archaeological Research (PAAR). He lectured on statistics at the Instituto Superior Técnico in Lisbon and is now an electrical engineer specializing in energy and control systems. He worked with Michael Hoskin on the orientations of two exceptional Neolithic tombs, Matarrubilla and La Pastora, near Seville, Spain. Luís Tirapicos, also a member of PAAR, is a history of science graduate student at the University of Lisbon. He studied astronomy at Porto University and currently is part of the Educational Service at the Museum of Science of the University of Lisbon, where he also researches the history of scientific instruments. His first work in archaeoastronomy was a collaboration with Michael Hoskin on the orientations of north Portugal megalithic tombs. Andrew Smith graduated in 1995 from the University of Adelaide with a Ph.D. in high energy astrophysics. He worked thereafter as an IT consultant specializing in scientific software development for several projects in astrophysics and archaeoastronomy and currently is part of the Pierre Auger Collaboration, working with the University of Adelaide on software and hardware development related to the Pierre Auger Observatory in Argentina. Abstract In this work we have conducted a study on the orientations in the landscape of twelve megalith- ic enclosures in the Alentejo region of southern Portugal. Some of these sites date back to the sixth or ifth millennium B.C. and are among the oldest stone enclosures in Europe. The results of the survey show a pattern toward eastern rising orientations. We used dedicated GIS software from one of the authors to produce horizon pro- iles and applied a statistical Bayesian approach in an attempt to check how the data would it to different models. In particular, we tested our results for a possible ritual interest in the Autumn or Harvest Full Moon and discuss previous studies by Michael Hoskin and colleagues on the orientations of seven stone dolmens of this area that have shown the existence of a possible custom for an orientation toward the sunrise. Resumen En este trabajo se realizó un estudio sobre las orientaciones en el paisaje de doce recintos megalíticos en el Alentejo (sur de Portugal). Algunos de estos sitios se remontan al sexto o quinto milenio antes de Cristo, es decir, se encuentran entre los recintos de piedra más antiguos de Europa. Los resultados de la encuesta muestran un padrón de orientaciones al oriente. Hemos utilizado un software de SIG dedicado de uno de los autores para producir periles del horizonte y aplicamos un tratamiento estadístico Bayesiano con la inalidad de comprobar cómo los datos se ajusten a diferentes modelos. En particular hemos probado nuestros resultados con un posible ritual de interés en la luna llena de otoño o Harvest Moon y discutimos los estudios previos de Michael Hoskin y colegas, acerca de las orientaciones de los dólmenes de piedra de esta zona que han demostrado la existencia de una posible costumbre de orientación solar. Despite various attempts to address the orientation of the megalithic enclosures in the Alentejo prov- ince of southern Portugal (Alvim 1996–1997, 2004; Silva 2000), it has hitherto lacked a comprehensive orientation investigation (Cardoso 2002:235) bring- ing together all enclosures, including those that have been completely dismantled. The Alentejo extends, roughly, from south of the Tejo River to north of the Algarve, a province on the