Andean astronomy refers to the body of astronomical knowledge and practices developed by the civilizations that succeeded one another in the Andean mountain range, from the first organized cultures to the Spanish conquest in the 16th century. This astronomical tradition spans over three millennia and encompasses many civilizations, each contributing to a corpus of knowledge transmitted and enriched from generation to generation.
The Inca Empire, although it existed for less than two centuries in its imperial form, represents the culmination of this long astronomical tradition. The Incas did not create their astronomy from scratch but inherited, assimilated, and perfected the knowledge of the civilizations that preceded them. However, it is under the Inca Empire that the most precise sources have reached us, mainly through the accounts of 16th-century Spanish chroniclers.
N.B.:
The Inca Empire (Tawantinsuyu in Quechua) developed between the 13th and 16th centuries, reaching its peak in the 15th century under the reign of Pachacutec. Inca astronomy was practiced by specialized priests and guided agricultural, religious, and political activities.
The Andean peoples developed a unique astronomy in the world, deeply rooted in their tripartite cosmological vision. Their meticulous observation of the sky over millennia allowed them to create a complex system of calendars, orient their temples with remarkable precision, and develop an original approach to constellations that distinguishes their astronomy from all other traditions in the world.
Unlike Western astronomy, which mainly observes bright stars, Andean astronomy placed great importance on dark constellations, those dark areas of the Milky Way formed by cosmic dust clouds. This astronomical innovation reflects a deep understanding of the night sky and a cosmovision where emptiness and darkness were as meaningful as light. The Andean universe was divided into three interconnected worlds: Hanan Pacha (celestial world), Kay Pacha (earthly world), and Ukhu Pacha (underground world), linked by sacred axes.
| Period | Scientific contribution | Precision or characteristic | Source or site |
|---|---|---|---|
| Caral (3000-1800 BCE) | Circular observatories | Monumental astronomical structures aligned with solstices, testifying to 5000 years of Andean astronomy | Caral-Supe, Peru |
| Nazca (200 BCE - 600 CE) | Astronomical geoglyphs | Lines several kilometers long aligned with solstices (June 21/December 21) and heliacal rising of the Pleiades | Nazca, Peru |
| Tiwanaku (500-1000 CE) | Gate of the Sun | Solar calendar of 290 days engraved in a 10-ton monolith, representing agricultural and ritual cycles | Tiwanaku, Bolivia |
| Inca Empire (1438-1533) | Ceques system | 41 radial lines connecting 328 huacas, functioning as an astronomical, social, and hydraulic calendar | Cusco, Peru |
| Inca Empire (1438-1533) | Dark constellations | Unique innovation in the world: observation of the dark zones of the Milky Way (Yacana, Yutu, Machacuay, etc.) | Observation from throughout the empire |
| Inca Empire (1438-1533) | Coricancha (Temple of the Sun) | Walls and windows precisely oriented on equinoxes and solstices, covered with 700 gold plates reflecting the sun | Cusco, Peru |
| Around 1450 | Intihuatana of Machu Picchu | Astronomical pillar projecting specific shadows at equinoxes, "shadowless" at solar noon during solstices | Machu Picchu, Peru |
| Inca Empire (1438-1533) | Observation of the Pleiades (Qollqa) | Climatic prediction based on apparent clarity in June, scientifically correlated with the El Niño phenomenon | Widespread practice in the Andes |
| Inca Empire (1438-1533) | Lunar-solar calendar | 12 lunar months (354 days) adjusted to the solar cycle (365 days) by direct observation of solstices | Inca administrative system |
| Inca Empire (1438-1533) | Solar towers (Sukanqa) | 12 towers on hills around Cusco marking monthly positions of the sun on the mountainous horizon | Cusco and major sites |
| Pre-Inca and Inca | High-altitude observatories | Astronomical and ritual sites at over 5,000 m altitude, offering exceptional atmospheric clarity | Andean peaks (Llullaillaco, etc.) |
| Inca Empire (1438-1533) | Concept of Mayu (Milky Way) | Cyclical vision of the celestial river linked to terrestrial rivers, reflecting understanding of the hydrological cycle | Andean cosmovision |
Source: National Radio Astronomy Observatory and Andean ethnoastronomical studies.
The observation of dark constellations (yana phuyu in Quechua, meaning "black clouds") constitutes the most original contribution of Andean astronomy. Rather than connecting bright stars, Inca astronomers identified animal shapes in the dark bands of the Milky Way. These constellations mainly represented animals important in daily and ritual Andean life.
These constellations were not just celestial markers but living entities participating in the cosmic order. Their seasonal visibility was closely linked to agricultural cycles and religious rituals, creating a celestial calendar that guided earthly activities.
The Incas called the Milky Way Mayu, meaning "river" in Quechua. This conception reflected their cyclical vision of water: the earthly river rose to the sky to form the celestial river, which in turn returned to earth as rain. This intuitive understanding of the hydrological cycle fit perfectly into their cosmovision where sky and earth were in constant interaction.
The Milky Way was divided into two branches by a dark zone, creating the image of a bifurcated river. These two arms of Mayu were associated with the two main seasons of the Andean agricultural calendar: the dry season and the rainy season. The orientation of the Milky Way in the night sky changed with the seasons, thus indicating the appropriate time for planting, irrigating, or harvesting.
N.B.:
The optimal visibility of the Milky Way in the southern hemisphere, combined with the high altitude of Inca sites such as Cusco, located at 3,400 meters above sea level, allowed for exceptionally clear observation of the dark structures.
The ceque system represents one of the most sophisticated achievements of Inca astronomy and engineering. From the Coricancha (Temple of the Sun) in Cusco, the capital of the empire, 41 imaginary lines called ceques radiated, connecting more than 328 sacred sanctuaries (huacas) located in and around the city.
The Coricancha itself was designed as an astronomical observatory. Its walls and windows were oriented along precise astronomical axes, allowing priests to track the sun's movement throughout the year and determine the dates of important agricultural and religious ceremonies.
The Sun, called Inti, held a central place in Inca religion and astronomy. The Inca (emperor) was considered the son of the Sun, and the Coricancha in Cusco was the main temple dedicated to this deity. The temple walls were once covered with gold plates reflecting sunlight, symbolizing the god's presence on earth.
The Incas meticulously followed the sun's annual movement by observing its rising and setting points along the mountainous horizon. They built inti watana (literally "where the sun is tied"), stone structures serving as sundials and observatories. The most famous is located at Machu Picchu, where a stone pillar was used to track the sun's position and determine the solstices.
During the winter solstice (June 21 in the southern hemisphere), the Incas celebrated Inti Raymi, the Festival of the Sun, marking the moment when the sun began its return northward, promising longer days and more favorable seasons. This ceremony was one of the most important of the year, involving sacrifices, offerings, and elaborate rituals intended to ensure the sun's return and the fertility of the land.
The star cluster of the Pleiades, called Qollqa (granary) in Quechua, played a crucial role in the Andean agricultural calendar. The Incas carefully observed the first appearance of the Pleiades at dawn (heliacal rising) in June, which coincided with the beginning of the dry season and marked the time for harvest.
The apparent size and clarity of the Pleiades in June were used to predict the climatic conditions of the coming year. If the cluster appeared bright and well-defined, it foretold a good season with normal rains. If the stars seemed faint or blurry, it announced a difficult year with insufficient or excessive rainfall, leading farmers to adjust their planting strategies.
This method of weather prediction based on the observation of the Pleiades has been studied by modern scientists who have confirmed a correlation between the apparent clarity of the cluster (affected by high-altitude cloud cover) and the climatic conditions resulting from the El Niño phenomenon in the Andean region.
The Incas used a calendar system combining lunar and solar elements. The year was divided into 12 lunar months of 29 or 30 days, totaling about 354 days. To align this lunar calendar with the 365-day solar year, they periodically added extra days, probably determined by direct observation of the solstices.
The Incas also built solar observation towers (sukanqa) on the hills surrounding Cusco. These structures marked the horizon and allowed astronomers to accurately track the sun's rising and setting positions throughout the year, thus determining the exact times for planting and harvesting for different crops and altitudes.
Inca architecture systematically integrated astronomical alignments. At Machu Picchu, several structures have precise orientations:
At Pisac, the archaeological site has a similar intihuatana and structures whose orientations correspond to key solar events. At Ollantaytambo, the Sun Temple has a window that is directly illuminated during the winter solstice, creating a light spectacle used in religious ceremonies.
These alignments were not merely symbolic but functional, allowing priest-astronomers to maintain a precise calendar without complex measuring instruments. The architecture itself served as a giant astronomical instrument, integrating the observational function into the very fabric of temples and palaces.
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