Several observations indicated that Aristotle's geocentric model, which posited that the Earth was the center of the universe and all celestial bodies revolved around it, was flawed. Here are a few key observations:
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Retrograde Motion of Planets: The apparent retrograde motion of planets, where they occasionally appear to move backward in the sky for a period before resuming their forward motion, was difficult to explain under the geocentric model. This motion was better accounted for in the heliocentric model proposed by Copernicus.
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Phases of Venus: Observations of the phases of Venus, made famous by Galileo, showed that Venus goes through a full set of phases similar to those of the Moon. This was incompatible with the geocentric model, which could not adequately explain why Venus would be fully illuminated when it is on the far side of the Sun.
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Stellar Parallax: The lack of observable parallax in stars was initially a challenge for heliocentrism; however, the inability of the geocentric model to predict this effect in the context of a moving Earth contributed to doubts about its validity. Eventually, astrometric measurements confirmed stellar parallax, corroborating the heliocentric model.
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Observation of Jupiter's Moons: Galileo's discovery of the four largest moons of Jupiter, which orbit that planet, demonstrated that not all celestial bodies revolve around the Earth. This challenged the notion of Earth as the center of the universe.
These observations, among others, ultimately led to the rejection of the geocentric model in favor of the heliocentric model, which correctly described the structure of the solar system.