Audio Guide：

Planets orbit around the Sun in different speeds. Planets closer to the Sun have a higher orbital speed whereas farther planets are slower. When observing planetary motions on Earth, sometimes planets are seen moving eastward in prograde, and sometimes in retrograde, or westward. Planetary retrograde motion could not be explained by the geocentric model. When Copernicus proposed the heliocentric model, simple and accurate explanations for planet retrograde were finally made possible.