One of the most complex cultures ever, was that of the Incas. To understand them, it is important to remember that the Incas were only the final phase of a series of cultures, which started building monumental architecture as far back as the third millennium BC. The Incas were therefore the heirs to a ancient legacy of knowledge and, also, of observation of the sky. The Inca empire was stretching from today's Colombia to Argentina. The state was called Tahuantinsuyu or "The Four Parts of the Earth". It was organised according to a centralised system, and ruled with the use of an official language, the Quechua. Governance of the country was in the hand of a rigid hierarchy, with on top the noble Inca families, residing in the capital, Cusco. The Incas did not have writings in the sense we usually mean, but information could be stored using devices, called Quipus, which consisted of clusters of ropes tightened to a main one and bringing different nodes and colors. In this drawing made in a Spanish chronicle, we see an Inca astronomer carrying a fork-like observation instrument and a Quipu, probably to register astronomical data. The Incas were extraordinary builders: many of their constructions employed the so-called polygonal technique, assembling huge blocks ‒ shaped like irregular polygons ‒ one on top of the other using apparently crazy, multi-angled, but perfect joints. A similar technique was developed much before also in the Mediterranean, and especially in central Italy, probably in pre-Roman times. Inca monumental architecture had a fundamental symbolic value: official constructions in stone – in particular ruler's palaces and estates – represented the materialisation of history, a way of asserting eternal rights for the ruler's family. Religion was a seminal, unifying element in the Inca state, as well as an effective instrument in the management of centralised power. In their cosmological vision, the Incas traced their origin directly to the Sun, which rose for the first time from a rock on a island of the Titicaca lake. The close relationship between rulers, gods and nature was reflected in the Inca religious life. The entire landscape was in fact considered as plenty of sacred things, starting from water, that had an especially sacred value. Water on Earth was in fact considered as directly connected with the water flowing in the celestial river, the Milky Way. The southern branch of the Milky Way is endowed with very brilliant stars, such as those of Scorpio and of the Crux-Centaurus constellations. The luminous band of the galaxy appears to us on the Earth as divided in zones, enclosing darker regions. The dark zones were interpreted by the Incas as images of animals forming “dark constellations”: among them a llama, a baby llama, a fox, a toad, a tinamou (means a turkey-like bird) and a serpent. The true heart of the Inca world was the capital of the empire, Cusco. The city ‒ as far as I know the only one of its kind in the world ‒ was planned in a manner that aimed to make it look, when seen from above, like the profile of a Puma, one of the sacred animals associated with Inca kingship. The feline is still today perfectly recognizable in the ancient city centre, with the huge hill called Sacsahuaman forming the head. Cusco was the navel of the world, and this concept was developed in the division of the town and its surroundings into radial sectors called ceques. Along each ceque there were many sacred sites or huacas, of various kinds, including stones, springs, and other places considered sacred for a number of reasons, for instance, caves. Some of the huacas are reported to be stone pillars located at the horizon and dedicated to the Sun. They were used to frame the Sun in special days of the year, and were therefore astronomical and calendrical devices. Unfortunately, and in spite of extensive research, the stone pillars of Cusco, or at least their foundations, have never been identified. However, unexpected discoveries have been made in recent years outside Cusco. Similar devices have in fact been identified in the Urubamba valley and also on the Island of the Sun. In both cases, they were meant for observations of the Sun at the winter (means June) solstice, the feast called Inti Raymi by the Incas. Further, we now know that the Andean tradition of solar observations using pillars on the horizon was already at least 1 600 years old by Inca times. This information comes from yet another Peruvian site, Chankillo, dated to the 4th century BC. Here are a massive, fortified temple and a large ceremonial area with several buildings. A natural hill in the shape of a rib traverses the site; on the hill, a row of thirteen rectangular towers was constructed. They are regularly spaced, and each has a staircase on both sides, so that it was possible to walk along the ridge, perhaps on the occasion of processions. From the temple, the towers were used as a device for observing the cycle of the Sun, which was seen to rise progressively behind each of them over the course of the year.
