日心说, or the heliocentric theory, is the astronomical model that places the Sun at the center of the solar system, with the Earth and other planets orbiting around it. This theory was first proposed by Nicolaus Copernicus in the 16th century, challenging the earlier geocentric model that positioned the Earth at the center. The heliocentric theory was further supported by the observations of Galileo Galilei and the laws of planetary motion formulated by Johannes Kepler. This model laid the foundation for modern astronomy and significantly changed our understanding of the universe and our place within it.