Kepler's Trigon — A diagram of great conjunctions by Johannes Kepler. While teaching, he had the striking realization that a circle was inscribed by the periodic occurrence of the conjunctions and that might be related to the orbits of the plants.
Photo Credit: By Johannes Kepler - De Stella Nova in pede Serpentarii (1606), Public Domain, https://commons.wikimedia.org/w/index.php?curid=16622989