‘The first Greek astronomer to suggest the Sun was the centre of the solar system was Aristarchus of Samos, around 290 BCE’
No one took him seriously, and most of his writings were lost. We know of him today primarily because Archimedes (whose writings do exist) referred to Aristarchus as holding this apparently nonsensical notion.
Aristarchus believed that the stars must be infinitely far away because they seemed motionless and the motions of the heavenly bodies could easily be understood if it were assumed that all of the planets, including Earth, revolved around the Sun. Copernicus knew of Aristarchus’ views and mentioned them in a passage in De Revolutionibus Orbium Coelestium that he later eliminated, as though not wishing to compromise his own originality.
Aristarchus reasoned that when the moon is illuminated by the Sun as a half-disc, the angle made by the line connecting the Earth and the moon and the line connecting the moon and the Sun would be exactly 90 degrees.
Accordingly, he constructed a pair of similar triangles, and found the relative distances for those two lines to have a ratio of 1:19. His method was correct but his crude instruments gave a false reading.
The correct ratio is also virtually the same ratio as the diameter of the moon to that of the Sun, which explains why during a total solar eclipse, we view the disc of the moon to fit almost precisely over the disc of the Sun.