The astronomer who improved on Aristotle's geocentric model and proposed a heliocentric model with the Sun at the center is Nicolaus Copernicus. His work laid the foundation for modern astronomy and shifted the understanding of the solar system.
If you're looking for the astronomer who improved specifically on Aristotle's model before Copernicus, that would be Claudius Ptolemy, who proposed a more complex geocentric model that included epicycles but ultimately maintained the Earth-centered view.
In summary:
- Nicolaus Copernicus: Proposed the heliocentric model.
- Claudius Ptolemy: Improved on Aristotle's geocentric model.