Copernicus, as far as can be reasonably known i.e. other astronomers may have been heliocentrists earlier than Copernicus but records of their thoughts either no longer survive or never existed in the first place...
And yet a reference in Archimedes' 3rd Century BC book 'The Sand Reckoner' describes the heliocentric theory of the solar system proposed by Aristarchus of Samos thus:
"His hypotheses are that the fixed stars and the Sun remain unmoved, that the Earth revolves about the Sun on the circumference of a circle, the Sun lying in the middle of the orbit..."
Yes indeed, but that's second-hand... The only surviving work by Aristarchus of Samos, "On the Sizes and Distances of the Sun and Moon", proposes a geocentric model...