The first scientist to significantly challenge the Ptolemaic model of a geocentric universe was Nicolaus Copernicus. While others had proposed heliocentric ideas before him, Copernicus was the first to develop a comprehensive and mathematically sound model of a sun-centered universe.

Here's why Copernicus is considered the pioneer:

* Helioentric Model: Copernicus's groundbreaking work, "De Revolutionibus Orbium Coelestium" (On the Revolutions of the Heavenly Spheres), proposed that the Sun, not the Earth, was the center of the solar system.

* Mathematical Support: He provided mathematical calculations and observations to support his theory, making it a more compelling alternative to the Ptolemaic model.

* Influence: While his ideas were initially met with resistance, they paved the way for future astronomers like Galileo and Kepler, who further developed and refined the heliocentric model.

It's important to note that while Copernicus was the first to create a comprehensive heliocentric model, other individuals like Aristarchus of Samos (in ancient Greece) had proposed a heliocentric view centuries before. However, their ideas lacked the mathematical rigor and observational evidence that Copernicus presented.