Today, we’re delving into the historical journey of understanding planetary motions and discovering that the Earth moves around the Sun. This realization was crucial in shaping modern physics and science. Initially, ancient astronomers observed that while most stars remained fixed in the sky, some stars, known as planets (derived from the Greek word for wanderers), exhibited unique, wandering paths. These planets included the Sun, Moon, Mars, Mercury, Venus, Jupiter, and Saturn. Planets generally move eastward (prograde motion) but sometimes appear to reverse direction (retrograde motion). Mars is particularly notable for its pronounced retrograde motion, which was pivotal in understanding planetary orbits. Ancient astronomers also noted that planets seem to reflect sunlight and hypothesized they were dimmer because they were farther from the Sun and Earth. This distinction prompted early scientists, known as mathematicians, to study celestial motions primarily for calendar creation and astrological practices. The Geocentric Model, championed by Aristotle (384-322 BC), placed Earth at the cosmos’ center. Aristotle argued this based on various observations, such as the absence of strong winds or shifting positions when objects fell to Earth, suggesting a stationary Earth. Claudius Ptolemy (circa 140 AD) further developed this model in his work, the Almagest, which dominated astronomical thought for over a millennium. He proposed complex systems of epicycles and deferents to explain planetary motions, with each planet embedded in a circular orbit within a larger celestial sphere. Although the Geocentric Model was complex and eventually proven incorrect, it provided accurate predictions and guided astronomical study until the 17th century. Next time, we will explore how the heliocentric model emerged and overcame the geocentric view, revolutionizing our understanding of the cosmos.