Question. Context: Heliocentrism is the astronomical model in which the Earth and planets revolve around the Sun at the center of the Solar System. Historically, Heliocentrism was opposed to geocentrism, which placed the Earth at the center. The notion that the Earth revolves around the Sun had been proposed as early as the 3rd century BCE by Aristarchus of Samos, but at least in the medieval world, Aristarchus's Heliocentrism attracted little attention -- possibly because of the loss of scientific works of the Hellenistic Era.

Question: who said sun is the center of the solar system?
Answer. Aristarchus of Samos
What step-by-step reasoning justifies that answer?
Reasoning: To answer the question, consider the following: The notion that the Earth revolves around the Sun had been proposed as early as the 3rd century BCE by Aristarchus of Samos, but at least in the medieval world, Aristarchus's Heliocentrism attracted little attention -- possibly because of the loss of scientific works of the Hellenistic Era.