On April 15, 1446, Filippo Brunelleschi, the overseer of the reconstruction of the Florence Duomo, died. His architecture and the machinery he designed to make it possible made a lasting impact not only on Florentine architecture, but inspired Leonardo's love and interest in machinery, physics, and engineering. The death of Brunelleschi also meant that someone else had to be hired to create and place the lantern, bronze ball, and cross at the top of the duomo. Verrocchio was hired for the job, meaning that Leonardo was able to work even closer on the duomo as he was an apprentice of Verrocchio.

Source: Mueller, T., & Yoder, D. (2017, September 19). Brunelleschi's Dome. Retrieved from https://www.nationalgeographic.com/magazine/2014/02/Il-Duomo/