日心说, or the heliocentric theory, is the astronomical model that places the 太阳 (Sun) at the center of the 太阳系 (solar system). This theory was first proposed by 尼古拉·哥白尼 (Nicolaus Copernicus) in the 16th century, challenging the long-held 地心说 (geocentric theory) that positioned the Earth at the center. The heliocentric model explains that the planets, including Earth, orbit around the Sun due to gravitational forces. This revolutionary idea laid the groundwork for modern astronomy and was later supported by the observations of 伽利略 (Galileo) and 开普勒 (Kepler), leading to a better understanding of celestial mechanics.