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abstract. The Thirteen Towers of Chankillo (fourth century B.C.E., coastal Peru) form a calibrated solar observatory whose visual span of 47◦ exactly matches the Sun’s annual declination range. This correspondence is not a coincidence of natural topography but the result of deliberate architectural design: the towers are positioned along a gently curving ridge at a fixed distance of approximately 235 m from two formal observing points, creating a “topological horizon” that compensates for the site’s latitude and the Sun’s oblique trajectory. While a companion paper explores the potential uses of this instrument for tracking long-period celestial cycles, the present article addresses a prior and more fundamental question: how did the builders arrive at this design, and what does the answer imply about the relationship between landscape, sky, and human cognition in the Early Horizon Andes? We develop three non-exclusive explanatory pathways. The first is empirical: the solar arc was measured over multiple generations and encoded in stone without knowledge of its astronomical cause. The second is cosmological: the Andean concept of pacha—a unified space-time continuum—may have motivated the materialization of perceived celestial order in the landscape. The third is geometric: the builders may have understood, through trial and error or through an intuitive grasp of projective geometry, how to compensate for the∼43.4◦ flat-horizon arc at their latitude by using the ridge height and the observer’s distance to stretch the visual angle to the required 47◦. None of these pathways is definitively established, but together they illuminate the depth of the enigma posed by Chankillo’s design and invite a broader reflection on the cognitive and cultural capacities of pre-Inca societies.