  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, \textit{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 non-local 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.
