Aristarchus of Samos

Aristarchus was the first known astronomer to propose a heliocentric model of the universe, placing the Sun at the center rather than the Earth. This concept was incredibly radical for the 3rd century BCE and was largely rejected by his contemporaries.

He used the geometric angle between the Sun, Earth, and Moon during a quarter moon phase to calculate the relative distances between them. While his methodology was sound, his instruments were imprecise, leading him to severely underestimate the Sun's actual distance.

Yes, Copernicus was aware of Aristarchus's ancient theories, though he initially hesitated to fully credit him in his early drafts. Copernicus eventually included a reference to Aristarchus in an early manuscript, though it was famously removed before final publication.

Only one of his treatises, titled "On the Sizes and Distances" (of the Sun and Moon), has survived to the present day. Unfortunately, the specific book where he detailed his heliocentric model has been lost to history.