The key event that took place during the Copernican Revolution, when most people started to believe in a heliocentric model of the solar system, was the publication of Nicolaus Copernicus's comprehensive heliocentric theory in 1543. This theory, published in his work "De revolutionibus orbium coelestium," proposed that the Sun, not the Earth, is at the center of the solar system, with Earth and other planets orbiting around it. This shift from the geocentric model (Earth-centered) to the heliocentric model (Sun-centered) marked the start of a broader Scientific Revolution. Additionally, the Copernican Revolution extended beyond Copernicus’s initial publication with further key contributions during the 17th century, such as Johannes Kepler’s laws of planetary motion—showing that planets orbit in ellipses rather than perfect circles—and Galileo's telescopic observations that provided empirical evidence supporting heliocentrism. These discoveries helped solidify the heliocentric model's acceptance among scientists and the broader public over time.

In summary, the event marking the Copernican Revolution was the publication of Copernicus's heliocentric theory in 1543, followed by significant advances by Kepler and Galileo that together led to the widespread acceptance of the heliocentric solar system model.