The following is quoted from The Incas: The Royal Commentaries of the Inca Garcilaso de la Vega, Orion Press, New York (1961), pp. 33ff. [Some of my comments are below the quotation.]
Not knowing how to write, the Incas had little knowledge of astronomy and of natural history. There were of course among them enlightened minds capable of philosophizing about subtle things, but since they were unable to write anything down, their discoveries disappeared with them...
They knew nothing about the Moon's phases, or the movements of the planets, with only three of which, the Sun, the Moon, and Venus, they were familiar...
The Incas understood the annual movement of the sun...they also knew about summer and winter solstices, and they left very visible evidence of this by building eight towers on either side of the city of Cuzco, four of which faced the rising, and four the setting Sun. Each of these groups of constructions comprised two small towers, about three stades in height [a stade is an old Spanish measure, the height of a man standing with arms outstretched], surrounded by to other, much higher towers. All these towers were separated regularly about eighteen to twenty feet one from the other. The tall ones...served to indicate the smaller ones, between which the Sun rose and set at the time of the solstice. In order to confirm the solstice, an Inca would take up his stand at both twilights, in a spot from which he watched the sun rise and set between the two towers. Since they counted the months by Moons, and since, too, the solar year was longer by twelve days than the lunar year, it was thanks to this observation of the solstices that they restored order to their calendar, according to the true, or solar, year, the improtance of which they had understood for reckoning when to sow their crops. These astronomical towers were still standing in the year 1560 [and the author in 1961 wonders if they are still standing].
They were also familiar with the equinoxes, which were the occasion for imporant ceremonies. At the March equinox, they cut corn in Cuzco...at the September equinox, the celebrated one of the four great Sun festivals, called Citua Raimi, meaning, the most important festival. They verified the equinoxes by means of a very richly carved stone column, which stood in the middle of the parvis of each of the Sun temples. When the equinoctial season was at hand, the priests made daily observations of the shadow cast by this column. A wide circle was drawn about the entire space of which this column constituted the center, and across the middle of this circle they drew a line from east to west, between two points which long experience had taught them how to locate. They could thus note the approaching equinox by observing the shadow of the tower on this line, and once this shadow was reduced by half, from sunrise to sunset, and at noon the column was entirely lighted up, without there being any shade, then they proclaimed that this day was the day of the equinox. The columns were immediately decorated with flowers and sweet-smelling herbs, and they set up a throne on the spot, for the Sun to come and sit in, as they said, "in all its light." On that day this heavenly body was adored with very special display, and it was also the recipient of gorgeous presents of gold, silver, precious stones, and other objects of great worth..
When the Sun set and they saw it disappear behind the sea, they said that it entered the sea, dried up a great amount of its waters and, like a very clever swimmer, dove under the earth and came out on the other side, which implies that the earth rests on the surface of the sea. They said nothing about the setting of the Moon or of any other stars.
[end of quotation]
The part about "not knowing how to write" is interesting. The Incas recorded at least business transaction records in knotted strings called "quipu" or "khipu". It seems there is some controversy about whether the khipu actually constituted a written language. Garcilaso de la Vega (quoted above) contradicts himself, saying here that the Inca astronomer/astrologers could not write, but in another place quoting the Inca version of what happened between Atuhualpa and Pizzaro, "as recorded in the quipus", a version that contradicts the version of the Spanish historians. Here is a review of a book on the khipu that discusses the question. That was in 2003. The author of that book is apparently the main researcher at the Khipu Database Project at Harvard. At that website you can learn more, but not the main answer as to whether the khipu constituted a written language or not, since that is still unknown.