Exactly 500 years ago, Nicolaus Copernicus drew a lattice of lines on a panel above the doorway to his rooms at Olsztyn Castle, then in the Bishopric of Warmia. Although its design has long been regarded as some kind of reflecting vertical sundial, the exact astronomical designation of the lines and related measuring techniques remained unknown. Surprisingly, Copernicus did not refer to his new observational methods in his principal work, De Revolutionibus. A data analysis of a 3D model of the panel has, at last, solved the mystery: Copernicus created a new type of measuring device—a heliograph with a nonlocal reference meridian—to precisely measure ecliptic longitudes of the Sun around the time of the equinoxes. The data, 3D model, and modeling results of our analysis are open access and available in the form of digital (Jupyter) notebooks.