The Topological Horizon of Chankillo: How a Pre‐Inca Civilisation Encoded the Solar Cycle in Stone

The Thirteen Towers of Chankillo (fourth century bce , Peru) form a solar observatory whose 47° visual span matches the Sun's annual declination range, achieved through deliberate architectural design along a curving ridge. This article addresses how the builders arrived at this design without knowledge of Earth's axial tilt. We develop three nonexclusive pathways: empirical (measuring the solar arc over generations), cosmological (the Andean concept of pacha as motivation for materialising celestial order) and geometric (compensating the ∼43.4° flat‐horizon arc via ridge height and observer distance to stretch the visual angle to 47°). This correspondence reflects a deliberate conceptual shift: from measuring the Sun's motion projected on the local horizon (azimuth, 43.4°) to measuring its true celestial motion (declination, 47°). The fixed observer distance (∼235 m) and the 13 calibrated gaps (∼0.5° resolution) transformed the ridge into a topological horizon—a precision instrument for reading the rhythms of the sky. None of these pathways is definitively established, but together they illuminate the depth of Chankillo's enigma.