Heliocentric theory is a model of the solar system that posits a central place for the Sun, with the planets orbiting it. It is most closely associated with the 16th-century work of Copernicus and the 17th-century work of Galileo, and the theory was widely adopted after Copernicus' death. Heliocentric theory replaced the older geocentric theory, which held that the Sun and other bodies orbit the Earth.
The geocentric theory held sway in Europe for many centuries, and its more elaborate versions described the observed motions of the heavens to within the accuracy of pre-16th-century observing techniques. Heliocentric theory was proposed to explain certain anomalies that emerged from a systematic observation of the sky. One problem that geocentric models had difficulty explaining was the apparent retrograde motion of some planets. The simplest explanation of this is that Mars, for example, orbits the Sun exterior to Earth, and that Earth periodically overtakes it in its orbit. A geocentric understanding, however, posits that Mars is orbiting a single point, which is itself orbiting the Sun. The unnecessary complications of geocentrism, combined with the direct observations of early astronomers, made a Sun-centered model increasingly attractive to theorists, despite vehement opposition from religious authorities of the time.